I'm under the impression that the fonts are now a bit smaller in the wordpress Visual Editor.
(I'm not asking for help, just ranting a little bit with hopes to convince you :) )
Of course it may depend on "local conditions", settings on my home computer.
This friendly rant is posted in case I am not mistaken and the Visual Editor actually switched to smaller fonts displayed with wordpress 3.3
To sum it up : enough with the fonts becoming smaller and smaller ! It's a real hindrance that we have to zoom in, more and more, for visual comfort. Small doesn't mean hype or techie friendly, it means "hard to see".
Making texts smaller goes against the evolution of the internet users' needs. As time passes we all buy larger and larger monitors and we can, at least, finally, un-stick our noses from the surface of our monitors, and sit at a comfortable distance from our monitors.
But, while we get farer and farer from the screens, the texts in several interfaces become smaller, that doesn't make much sense, and forces us to adjust more and more the zoom level in several websites (recommendation, BTW, the Firefox extension called "default zoom level", it remembers the per-site settings.)
(The users of cellular phones and of tablets are still a small minority.)
See what I mean ?
In this regard, making the texts smaller in the Visual Editor goes against the natural flow of internet uses :)
How about making it larger with wordpress 3.4 ? ;)
(and there's still the possibility that it's only happening on my computer, and then, sorry to have wasted your time)