If my opponents body is heavier, I have to spend more energy to make it move- however my opponent also has to spend more energy to make it move. The problem is that my opponents body is more use to moving that weight, because it has to move that weight every day, and it's super use to moving that weight if my opponent actually lifts and does enough cardio.

If you're fighting an opponent that is well adapted to their frame and it's characteristics, you're going to be at a disadvantage- because what is every second of life to them is heavy to you. If you could move your smaller, skinnier frame as well as they move their bulkier and heavier frame, then the best possible build would be lean.

Any strength athlete will tell you that they get weaker on a cut, and it's more difficult to maintain strength on a cut. This is because a more muscular and fat person is always in a caloric surplus- so for them it's more about supplying the necessary energy for that bigger bulkier frame. There is also something known as momentum, and the heavier an object is, more energy is required to stop that momentum.

If someone is lean, then they in theory have less weight slowing them down and have a much greater potential to accelerate their movements. However, if they haven't built muscle on that lean frame and/or programmed that lean frame with progressive overload sufficiently- then they will be at a disadvantage in a fight because they aren't explosive enough.

So why don't fighters look like bodybuilders? Because the more muscle you add onto a frame and the more drugs you take, the harder it is on your other bodily systems. Those muscles need a tremendous amount of oxygen to run well and if you haven't built the heart and lungs necessary to support them, your bodybuilder fighter is going to gas out far too quickly and won't be able to get their body to move.

Take everything I just said and then look at the physique of your average UFC fighter.