[R-SIG-Mac] sizing of quartz and x11 windows

[Parlamis Franklin]

whenever i open an x11 or quartz window, the dimensions of the window  
are roughly 74% and 72%, respectively, of what i call for.  for example:

quartz(width=3, height=4) leads to a window that is roughly 2.15  
inches by 2.9 inches, and
x11(width=3, height=4) leads to a window that is roughly 2.21 inches  
by 3 inches

i am using dual apple cinema 23" displays set at the 1920x1200  
resolution.

can anyone point me in the direction of understanding why this is  
happening?