I think a lot has to do with weight, when I zed fared from HKG to SFO on Cathay Pacific they would weigh the carry on's, they limited them to something like 8 kilo's. I had a small rollerbag (one that fits in the regional overheads) and they did not even ask to weigh mine. I was lucky because my bag was way over the limit for weight. Also, I think a lot of non U.S. airlines only allow 1 carry on, regardless of size.

Tony