What small window on the Option bar? I have not found a method to change the size of the temporary dimension font sizes from release 4.5 to the current release 9.0. There is no display for the values in a temporary dimension on the option bar or the status bar below.
Could you include a picture with your post reply indicating where in Revit's interface or on the Option bar you can see the value for a temporary dimensions value other than on the screen.
In answer to the original question, Why are some temporary dimensions text values displayed larger, probably because the monitor size is larger or the screen resolution is set to a lower DPI value (i.e. 1024X768 VS 1280X1024.
Mel Persin, AIA