That looks as it does for me as well. It's a bitch to pick apart gmail, so I can't be bothered to check how they're defining font-size, but i know there have been som differences with regard to this previously. If I were to guess I'd say that the Firefox/Mozilla team has consciously mimicked the IE behavior, while Opera has implemented a spec and it has ended up a little different. Could be a rounding difference from something that's ambiguous in the spec, or it could be a bug in one of the browsers.
There has been a definite design philosophy difference between Opera and Firefox when it comes to this. Firefox have to some extent focused on ease of transition, while the Opera people have stated that implementing some IE-like rendering path in quirksmode is not something they're going to do.
You can either use the zoom feature, or bump the default text size up one notch under Preferences, Web Pages tab, Normal font button, but if you want it to look *exactly* the same as IE you're using the wrong browser.