Thats because they are different weight classes. And do damage and have speed, accordingly to the weight classes.

Example:

A Lightweight with 100 power

Of all the power possible in the game(from 0 to 100) Would probably be at 60

But a Heavyweight with 100 power

Of all the power possible in the game(from 0 to 100) would be 100

But a Lightweights Speed at 100

Of the entire game possibility about a 90

But a Heavyweights speed at 100

Of the entire game possibility at about a 70



It's not going to max out because of your boxers weight class, and it will only be as high as that weight class allows, so every fighter is different.

If they left all the stats at maximum the same we'd have Heavyweights that move like Lightweights.

Do you get what I was trying to say?