Just nevermind the ball! Just do all you can with one size, and if you have to use the other, it'll just take like 1-2 days to get used to it. Not a big deal at all.
However,
If you guys really wanna talk about that matter from the root, it all began with gravity. If you guys ever taken any physics class, you guys probably heard of "g", the acceleration due to gravity. Well, guess what? On Earth, "g" is the same everywhere, any time! It always equals to 9.81 m/s^2. That means everything, no matter what size (unless the drag due to air resistance is so high) will always fall at the same rate.
Now because the balls have relatively the same sizes (one is 4.5 the other 5) it's not a big deal of difference in the falling of the ball.
OK, when we're clear of that, other parameters can be considered,
Like LIS said, he had "unnecessary" big and predetermined revolutions around the ball, and because of that, it makes complete sense why he prefers size 5 for combos. The surface area of contact of a size 5 ball is larger than that of a size 4.5, and because all things fall at the same rate, and LIS's leg is always moving in a constant predetermined path, maximum contact = maximum ability to control the ball. That's why he prefers size 5 for long combos.
For the guys who don't have the "predetermined" path of revolution like him, it'll be beneficial to use a size 4.5, because you can cheat your way around the revolution. Since the balls are falling at the same rate, one with a smaller area will be easier for your leg to get around. The matter of the size of the revolution is more at work here, and less for the surface area.
The point I'm tryna make here is that, because everybody's different, you can't say that one can do better tricks with a 4.5 ball without looking at his revolution (which complicates matters because we don't have time to do that for like everyone), and even if so, cheating your way around the revolutions isn't nice! It's just better to just forget all about it and focus on learning your tricks. Any kind of ball that you want should do! After all, "g" is constant!
And so, that's taking matters of the balls from the root. If you understand it, great. If not, just follow my advice above: "Nevermind the ball! Just do all you can with one size, and if you have to use the other, it'll just take like 1-2 days to get used to it. Not a big deal at all."