To reproduce:
- Add four buttons to a RadForm
- On each button click use ThemeResolutionService to change the theme
=> the form size increases in height

Workaround:
Set the MaximumSize of the form to the current form size prior changing the theme and then remove this setting:
private void radButton1_Click(object sender, EventArgs e)
{
    this.MaximumSize = this.Size; ;
    ThemeResolutionService.ApplicationThemeName = "Office2010Blue";
    this.MaximumSize = Size.Empty;
}