And a 757 comes with different pay rates than an 767. Take CAL who flies the two types as a common fleet, each aircraft pays differently. "Soft Time" for training, vaca, and the like is based on a blended rate computed on the number of aircraft in each fleet.
Your 772/744 example, the only US carrier that matches is United, who pays a single "widebody" rate for both aircraft. That rate is calculated with the knowledge that the 74 is bigger, for negotiation purposes a sort of blend is figured. UA's 777s vary in capacity, but on average they have 84% as many seats as their 744. The next step of capacity reduction, to the 767, is about 83%, and one finds a new payscale.
The 50 seat RJ has only 67% as many seats, and the rate for that aircraft was not negotiated with larger aircraft in view. Heck, it wasn't negotiated at all, just imposed with a member vote for eyewash purposes. Much like putting a dummy temperature knob in front of an FA, it makes 'em feel content.
Saying that mainline doesn't consider aircraft size in the way SkyWest did is preposterous.