If for an unknown reason your Internet Explorer seems to show things bigger than Firefox, and you’ve checked that it’s not the Text Size, it may be because of your screen DPI settings. The most obvious symptom is when images in IE appear larger and low quality (jagged edges) due to the enlargement.
If so, open your Display Properties, choose “Advanced” under the “Settings” tab. The view here might vary by display card, but look out for an option for DPI settings. For my case I was having 120dpi instead of 96dpi. Once I changed it back and restarted the machine IE was doing fine. However doing so makes my other screen fonts too small on the high-res monitor.
To fix that you’ll have to manuall adjust the fonts in the Display Properties again. I was too lazy so I switched back to 120dpi again, and stayed with the IE tab plugin…