Luke listed as 86kg. TMM 87kg.
Kalyn Ponga 92kg,
Shaun Johnson 91kg, Nathan Cleary 92kg
There is a range of weights for halves in the competition, as there always has been.
For example, even just back a few years some of the greats like Brett Kenny, Kevin Walters, Terry Lamb were lighter than Luke is currently.
Generally it is about technique and attitude if they are going to be reasonably successful at defending in the line - SJ and Ponga are basically the same weight. Being a good fullback doesn't always translate to being a good 6 or 7. The defensive workload is a lot higher and having to make 20 tackles front on at a high effective rate is a lot different to 4 or 5 attempted and making 1 or 2 of them.
Luke's injury wasn't from his defensive workload though, just an unfortunate incident. Problem he has is that at 25 years old he hasn't been able to put together a consistent run of games at a high level ie NSW cup/NRL. Been plenty of players like that over the years. Hopefully he can come back stronger and his body can stand up to the rigours of the game.