All sizes in dialog templates are indeed specified in "dialog units", which are based on the metrics of the dialog's font. So in theory if you are to double the font size, all dialog elements would just double in size and all will be well.
In practice, this is not sufficiently accurate to accommodate text size differences when going from one font/size to another. It's just too crude.
For example - http://imgur.com/STtd8OQ
Top is SegoeUI/9pt, standard for DPI of 96.
Middle is SegoeUI/10pt, standard for DPI of 125.
Bottom is Tahoma/8pt, standard for XP at 96.