The 15" Air weighs 270g more than the 13" Air, that's an extra 22%. In a daily commute bag that makes a noticeable difference. For me, the smaller footprint of the 13" is also much more pleasant and less unwieldy – on the couch, in the car, in an airplane etc. For desk-only use I'd probably get a big fat 16" MBP. Or a mini
Now, I've lived with large laptops for many years, I had the 17" unibody MBPs and the aluminium one before that. It's not like I couldn't handle those. But times and priorities change. Comparing the 13" and 15" Airs side by side at the Apple Store I had no doubt that the 13" was the right size for me. If both cost the same it would have been a tighter race, but with the 15" being substantially more expensive the 13" was the clear winner for me.