It's hard to describe, but the rendering of my 2010 13" MBA on a Dell 2412m looks larger than it should be for 1920x1200. It looks like it's rendering at 800x600 or something like that. Using a PC, everything looks smaller like I would expect at 1920x1200 resolution.
Go to System Preferences and check how you set up the monitor resolution. If it is set to 800x600 in System Preferences, then of course it will render 800x600, so you just change it to something reasonable.
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I have a MBA (2010 model) connected to a U2412 via mini-displayport to DVI adapter, then then DVI cable to the monitor. I'm experiencing the same thing but can't figure out precisely why. It just seems....less sharp then it's supposed to be?
Cheap mini-dp to DVI adaptors can cause problems. But most likely you have selected the wrong resolution in System Preferences for your monitor. It may be because the Mac figured out that a cheap adaptor couldn't handle the highest resolution. That would make the image unsharp.