Workaround: set the AutoScaleMode property of UserControl to None.
This is not an issue, it's just that RadHostItem inherits the font from QuickAccessToolbar and sets it to the hosted control(UserControl). The default value of UserControl's AutoScaleMode property is AutoScaleMode.Font. In the described case the font is bigger than the initial one and this is why all the controls in the user control are scaled. To preserve the size of all controls in the UserControl set the AutoScaleMode property to None.