Im using xubuntu 8.04 on a Thinkpad X60T Tablet PC and therefore i use to rotate the screen sometimes (with xrandr).

When i start a new gtk-app after i rotated the screen, all its font-sizes increase significantly.
When i switch back to the default rotation and restart the application the font sizes are normal again.

Adjusting the dpi-settings does not seem to make a difference, i had it on system-default, 96dpi and 145dpi which had no effect.

In Gnome or other WMs/DEs the font sizes are not affected by the screen-orientation.

I dont know to which component this belongs to and just took a guess.