Yeah... strength would be low in the lower weight classes, quickness would be low at heavyweight. Course there's always got to be the wild exception, there are a few heavyweights that are really fast, just not so many. Maybe have an accepted range for each weight class with a 1 in 10 chance of a guy having extra in a particular area, or for having less than normal? Thus my big strong fighter may have terrible stamina while little guy has unusual strength but is otherwise ordinary.
I would think technique would be uniform for all classes and people could have any amount at random. Stamina and endurance are a little tough to call, perhaps it should be random with some thought given to weight, as KnockOut suggested. Strength, agility and quickness need to have their limits, particularly strength - a guy either has power or he doesn't, even building muscle doesn't necessarily give you that power.
Weight would need to be a big factor in agility... a guy who weighs 300 just can't move like a guy who weighs 125. You should set that up relative to weight classes, but in the heavyweights make it a sliding scale so that the heavier he is the slower he moves...of course with the rare exception.
Aren't you glad you're getting it from two sides?
Having these limits will make for more explosive fights I just realized. Heavyweights who can't get out of the way will get knocked out easier, today I can have the most agile heavyweight in the world regardless of his weight.