Shifting font sizes in KDM

Hi there, I am using the graphical KDM login on SuSE 7.2. For some reason, the font size shifts between normal and large (typically unix-style ugly scaled font) between sessions. When I start a KDE session, the size shift remains visible. Other windowmanagers (WindowMaker, Enlightenment) seem unaffected. The problem seems unrelated to the version of either X (4.0.3 or 4.1.0) or KDE (2.1 or 2.2). I run X on a Matrox G400 DH graphics card with the glx and drm modules enabled. Does anyone has any idea what causes this and how to prevent it? Tim