I find it a little better for higher density routes to be flown more often with smaller aircraft
Dallas-Houston has a lot of business travelers from both cities that will go to the other for the day for business, those routes are flown more often with smaller aircraft(Md80,757,727,crj,737) so that way if joe who lives in houston has to go to dallas for the day,he can travel on a smaller jet but he will be able to find a flight that suits his times,if they were served with 747's,767's and 777's,more than likely he would have to spend the night in dallas or something like that. but if i did have to choose a couple of routes,
YYZ-YVR and YUL,
just a couple....
Give me a break, I created this username when I was a kid...