Most trapways are 2-1/8" with the larger (3" or more) flush valves.
That has been a good size.
Some companies are using slightly bigger valves and trapways, mostly so they can say "bigger".
Toto was about 15 years ahead of the others in coming out with the new design that has just recently been copied by others.
Is bigger better, or is it marketing hype?
There isn't that much difference between 2-1/8" and 2-3/8" to make much difference. You're talking 1/4".
If you have waste leaving your body that is bigger than 2" in diameter, then maybe it does.
Most people don't.
The MaP testing is based on 7/8" in diameter waste.
Both toilets use siphon flushing. Too much size, and you loose siphon. Otherwise there wouldn't be a limit on size.
The biggest flushing problem I"ve had with a Champion in a friends home, was when the trip lever starts to wear in and it doesn't let enough water out of the tank to complete a flush. That has nothing to do with size though.
I replaced the tank on that one, and so far it's doing the job. That also fixed the the leaking seal problem.
I don't like how shallow the bowl is though. The deep part doesn't seem placed right.
If size does matter to you, and it could with certain medical conditions which results in large than normal bowl movements, you could consider something in the 3" to 4" range, which the Caroma toilet uses. The Caroma uses a Wash down flush.
The Caroma doesn't flush as much miso paste as the others at one time, but with the larger openings, you can drop bigger things through it.
That all being said, most of the new toilets that use a 3" or larger flush valve and a trapway of 2-1/16" or larger will flush better than previous versions.