To reproduce:

1. Add RadDropDownList and a RadTextBox to the form at design time.
2. Drag the VisualStudio2012Light from the toolbox and drop it onto the form.
3. Use the following code and run the application:

public Form1()
{
    InitializeComponent();

    ThemeResolutionService.ApplicationThemeName = "VisualStudio2012Light";
}

You will notice in the attached screenshot that the RadDropDownList's height is greater than RadTextBox's height.

Workaround:           this.radDropDownList1.DropDownListElement.EditableElement.TextBox.Padding = new Padding(-2);