It's not about weight, it's about fitness, and one component of being fit is to have relatively low body fat, because fat is not very efficient, whereas muscle is.
Body language is a very powerful tool. We had body language before we had speech, and apparently, 80% of what you understand in a conversation is read through the body, not the words.
It's not magic! It's physics. The speed of the turn is what keeps you upright. It's like a spinning top.
Because muscle is heavier than fat, dancers weigh more than you might think, but they are usually very lean.
As you get older, the physical abilities decrease, which is particularly frustrating because your brain gets so good! So as you are becoming less technically or physically able, younger dancers are emerging who need the space to perform.