My own opinion is that it doesn't have as much to do with size as much as it has to do with the weight of the wheel/tire/tube combo. A 700x23 or 25 is going to be significantly lighter than a 26x2.125. I went from 26x1.5, standard tubes (LOTS of flats ) to 26x2.125 (still lots of flats, but better ride) to my current 26x2.125 with slime and puncture resistant tubes. I don't get any flats anymore. BUT, its a VERY slow tire/wheel/tube combo. For commuting its great though, I can still do a leisurely 12-14 mph and be assured that no matter what I'm not going to get a flat, short of something slashing my tire. If I were worried about going faster, I'd get a road bike with slim tires and probably do 18-19mph instead of 12-14. If I want to work out though, I can get my fat tire ride up to 16mph consistently, it just takes alot of effort to accelerate those heavy tires and tubes.

PS- I am seriously considering going to 26x1.25 on my fat tire commuter bike, so that I can fit fenders with plenty of room to spare. Even with puncture resistant tubes (or slime, not both tho) it'll be a fair bit lighter than what I've got, and the bike will go faster.