Overestimation of TextBounds!?

1
Dear Psychtoolbox experts,

I'm trying to estimate the pixel size of 'x' by Screen('TextBounds').
Unfortunately, even though the aspect ratio of 'x' is roughly 1 on my computer screen,
the returned x-height is almost 2.5 times larger than the returned x-width.

For example, when I run the following code:

===
clear all; close all;
w=Screen('OpenWindow',0,[0 0 0]);
SCREEN(w, 'TextFont','Times Roman');
SCREEN(w,'TextSize',100);
Screen(w,'TextBounds','x')
Screen(w,'DrawText','x',500,500,[255 255 255]);
screen(w,'Flip');
pause;
Screen('CloseAll');
==

I get [0 0 53 133] as the bounding box of 'x'.
Am I making any mistakes here?
My environment is Mac OSX 10.4.11 + Matlab 7.4.0 (R2007a) + Psychtoolbox 3.0.8.

Thanks!
Tren
--
Tren Huang, Ph.D.
Research Associate
Vision Sciences Lab
Department of Psychology
Boston University