Just like other sports, there is an "ideal" height/weight.
From gymnastics to football to track to swimming to tennis to whatever, it doesn't matter.
Though there will always be outliers, the needs of the sport in terms of physical traits (strength, agility, cardio, flexibility, etc) will always have a certain bell curve
And this bell curve, expectedly, will be different than other sports.
Combat sports are different only in that they have weight classes - this is to allow competition, as without weight classes it's understandable that the bigger fighters would dominate the smaller, generally.
But even with no weight classes, there would be the law of diminishing returns when it comes to size. As several pointed out, the heavier you get, the stronger you may be, but then cardio, agility, flexibility, etc will tend to decline. So at some point you can be too big, just like it is for other sports.
Even though we'll never know, I'd speculate that in an alternate universe, where there were no weight classes, eventually the "best" fighters would be between 185-230, in terms of min/maxing height, reach, strength, KO power, speed, agility, cardio, etc...
For those that seem to hold that (American) Football are then end-all-be-all physically, are blind to the fact that they are ideal build for short bursts only. They literally go full on for 10 seconds, and then rest for 30. And that's what they are built for. I do not believe that it means they would automatically be the best at MMA, had they "do MMA instead of football at young age". I also believe that they would have been built differently had they chose that path.
Football nut huggers forget about that aspect of it.