OK, thanks, I found that.
Now I checked and you have a field 'image_use_side' for each theme. It was set to 'longest' for my theme but was on 'height' for the default theme.
Is seems to use the value for the default theme also in another theme to create the thumbs because I had the trouble here and now that I set it to 'longest' also in the default theme the thumbs are back to the correct size (also in the theme I use)
Also, for some reason, the check (for image_use_side) in the admin/options/themes tab can only be changed when you use the default theme. If you change it in another theme, it reverts to the value set in the default theme.
Maybe I did something wrong while adapting the default zenpagetheme to my likings but I find this curious behavour.
Many thanks in advance,
Filip