The MBA might be small and light but there's nothing lightweight about its performance. In the past 5 years I've moved from a 15" MBP to a 13" MBP to a 2011 13" MBA and I'll probably trade this one in on the new model to get its improved battery life.
There's nothing I can do on my iMac that I can't do on my MBA. Two caveats: 1) I'd really rather use Photoshop and InDesign on a large display than the 13" display and 2) the discrete graphics card of the 15" MBP does make a difference with some tasks. But for coding, unless you are doing 3D graphics related coding, that isn't going to be an issue.