Horwill is listed on Wikipedia as 117kg. He has no issue being lifted overhead. Skelton is listed as 135kg. Thats 9kg more for each lifter, and around 70kg each. I doubt the increase makes a difference to the lifters, and I also doubt a professional player lacks the strength to throw up significantly more weight than 70kg. Will is probably too big to jump with a single lifter, but that is not uncommon.
The issue is more likely speed of the jump, which is driven by his power production. This should be trained.
It's not that simple.
The modern lift is based a lot on the jumper "springing" off the ground, and then instantly going stiff as a board. Some are better at this than others, and I suspect that it's not something that can really be trained to a great level, like fast twitch muscle fibres.
The front lifter is the main lifter, holding onto the jumper's thighs. Hence the thigh strapping on lineout jumpers. These guys do the harder part of the lift.
The back lifter primarily provides stability, with their hands either under the backside, or on the upper thighs (although this is not a stable lift at all).
Smaller jumpers who can spring quickly off the ground are significantly easier to lift, and may even get higher as their spring is better, allowing the lifting players to fully extend. If the spring isn't great, the lifters will struggle to extend. Trust me, lifting behemoths who don't spring well will really take it ouf of you, profressional player or no.
So you can see why large guys with lower centres of gravity are just an absolute bitch to lift, of which Skelton is one. He'd be a part time guy at best, maybe gone to less than once a game. If any decent opposition jumper marked up on Skelton with a good lifter or two, then they would get in front, hence why Skelton would never be a key target.
