Very good point - however I think this is just an implementation detail of Git as it stands today.

Also, if the person is working with SVG (for example), then it's less of a problem.

Also, given the cheapness of disk, I don't think it would be a limiting cost. And since git is open source, if I were to actually make this thing, it would definitely incentivize me (or others) to make git less bad for storing binary files :)