Why does the text overflow?

Website:
http://www.macdear.com

Could somebody please tell me why this page flows reasonably well in FF
when increasing browser text size up to 2 times (Ctrl-+), but in IE the
main part of the screen drops down below the menu when the text size is
changed from 'normal' to 'large', as if the holding div isn't big enough?

Thanks,
Harvey