Reset by Clear, RemoveRange causes a memory leak of GridViewRow instances (or TreeListViewRow in RadTreeListView).
To work this around, you can call the Clear method of the RadGridView's Items collection and then reset the ItemsSource property.

this.radGridView.Items.Clear();
this.radGridView.ItemsSource = null;
this.radGridView.ItemsSource = newSource;
Edit: The number of instances of the GridViewRow class are relative to the number of containers which cover the visible viewport. They are kep by internal caching mechanism used for faster reuse of containers is data-reload scenarios. Containers are dettached from their previous viewmodels (DataContext).

In non virtualized scenario, the number of the live containers is the same as the number of items in the GridView's ItemsSource and when changing the source, the number remains the same as the items in the source.

We are closing this issue as we think the mentioned internal cache shouldn't be considered a memory problem but rather a powerful performance optimization internal tool for extensive data-refresh scenarios.

The current workaround is to set the FontSize of the underlying RadComboBox explicitly:

		<Style x:Key="RedCellStyleStyle" TargetType="telerik:GridViewCell" >
			<Setter Property="FontSize" Value="25" />
			<!--workaround-->
			<Style.Resources>
				<Style TargetType="telerik:RadComboBox">
					<Setter Property="FontSize" Value="25" />
				</Style>
			</Style.Resources>
		</Style>

As a workaround, the Path with the PathGeometry setter in the style can be replaced by a RadGlyph with a pin icon. The IsPinned Trigger in the style can be replaced with a DataTrigger to the parent row's IsPinned. Here's an example of the updated style for the Fluent theme:

<Style x:Key="GridViewPinButtonStyle" TargetType="grid:GridViewPinButton">
        <Setter Property="PathStyle" Value="{StaticResource PinUnpinPathStyle}"/>
        <Setter Property="Foreground" Value="{telerik1:FluentResource ResourceKey=IconBrush}"/>
        <Setter Property="MinHeight" Value="28"/>
        <Setter Property="MinWidth" Value="22"/>
        <Setter Property="IsBackgroundVisible" Value="False"/>
        <Setter Property="Command" Value="{x:Static telerik:RadGridViewCommands.TogglePinnedRowState}"/>
        <Setter Property="CommandParameter" Value="{Binding}"/>
        <Setter Property="SnapsToDevicePixels" Value="True"/>
        <Setter Property="BorderThickness" Value="0"/>
        <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
        <Setter Property="VerticalContentAlignment" Value="Stretch"/>
        <Setter Property="FocusVisualStyle" Value="{x:Null}"/>
        <Setter Property="IsTabStop" Value="False"/>
        <Setter Property="Padding" Value="0 0 2 0"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="telerik:RadPathButton">
                    <Grid Background="Transparent">
                        <VisualStateManager.VisualStateGroups>
                            <VisualStateGroup x:Name="BackgroundVisibility">
                                <VisualState x:Name="BackgroundIsHidden">
                                    <Storyboard>
                                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="OuterBorder" Storyboard.TargetProperty="Visibility">
                                            <DiscreteObjectKeyFrame KeyTime="0">
                                                <DiscreteObjectKeyFrame.Value>
                                                    <Visibility>Collapsed</Visibility>
                                                </DiscreteObjectKeyFrame.Value>
                                            </DiscreteObjectKeyFrame>
                                        </ObjectAnimationUsingKeyFrames>
                                    </Storyboard>
                                </VisualState>
                                <VisualState x:Name="BackgroundIsVisible"/>
                            </VisualStateGroup>
                        </VisualStateManager.VisualStateGroups>
                        <Border x:Name="OuterBorder"
                                BorderThickness="{TemplateBinding BorderThickness}"
                                BorderBrush="{TemplateBinding BorderBrush}"
                                Background="{TemplateBinding Background}"
                                CornerRadius="{TemplateBinding CornerRadius}"/>
                        <telerik:RadGlyph x:Name="UnPinIcon" Foreground="{TemplateBinding Foreground}" FontSize="16" HorizontalAlignment="Center" VerticalAlignment="Center" Glyph="{StaticResource GlyphPin}"/>
                    </Grid>
                    <ControlTemplate.Triggers>
                        <Trigger Property="IsMouseOver" Value="True">
                            <Setter Property="Foreground" Value="{telerik1:FluentResource ResourceKey=AccentMouseOverBrush}"/>
                        </Trigger>
                        <Trigger Property="IsPressed" Value="True">
                            <Setter Property="Foreground" Value="{telerik1:FluentResource ResourceKey=AccentPressedBrush}"/>
                        </Trigger>
                        <DataTrigger Binding="{Binding IsPinned, RelativeSource={RelativeSource AncestorType=telerik:GridViewRow}}" Value="True">
                            <Setter TargetName="UnPinIcon" Property="Glyph" Value="{StaticResource GlyphUnpin}" />
                        </DataTrigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

ArgumentException: 'RadGlyphExtension' is not valid for Setter.Value. The only supported MarkupExtension types are DynamicResourceExtension and BindingBase or derived types.

StackTrace:
   at System.Windows.Setter.Seal()
   at System.Windows.SetterBaseCollection.Seal()
   at System.Windows.Style.Seal()
   at System.Windows.Style.Seal()
   at System.Windows.StyleHelper.UpdateStyleCache(FrameworkElement fe, FrameworkContentElement fce, Style oldStyle, Style newStyle, Style& styleCache)
   at System.Windows.FrameworkElement.OnStyleChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
   at System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
   at System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
   at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
   at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType)
   at System.Windows.DependencyObject.InvalidateProperty(DependencyProperty dp, Boolean preserveCurrentValue)
   at System.Windows.FrameworkElement.UpdateStyleProperty()
   at System.Windows.FrameworkElement.OnInitialized(EventArgs e)
   at Telerik.Windows.Controls.RadButton.OnInitialized(EventArgs e)
   at Telerik.Windows.Controls.RadPathButton.OnInitialized(EventArgs e)
   at Telerik.Windows.Controls.GridView.GridViewPinButton.OnInitialized(EventArgs e)
   at System.Windows.FrameworkElement.TryFireInitialized()
   at System.Windows.FrameworkElement.EndInit()
   at MS.Internal.Xaml.Runtime.ClrObjectRuntime.InitializationGuard(XamlType xamlType, Object obj, Boolean begin)

ArgumentOutOfRangeException is thrown when performing a CollectionChange with Replace action on the underlying collection and there's an applied FilterDescriptor.