I think its just from the confidence most people get knowing they are riding on a light uni.
With a lighter frame, you don’t have to use a much exertion as you would to jump on a heavier uni. You use less energy on a lighter uni to do the stuff you can on a heavy uni. But with the unis people use today, the difference in weight is only by 2-3 pounds at the most. Which hardly makes a difference in how they perform, just how they feel.
I have done that, kinda. I do trials rides with my 2-liter camelbak on sometimes, and that doesn’t affect my jumping like you mentioned it wouldn’t. It felt weird having 2 litters of water strapped to my back while I was jumping, but I got over that on the 2nd trials line.
