One reason that spine players are generally lighter than everyone else isn't because there's some rule requiring it. The reason is that they make the most movement off the ball of every player on the field and this is extremely fatiguing for the bigger guys. These movements also require some vision and game smarts that aren't really a factor when forwards and outside backs are chosen for development as first graders. I mean, it's good if they have those attributes at a high level, but there's so much that is required physically from forwards and outside backs that their ability to play in the spine isn't prioritised, either in recruitment or training. Halves and fullbacks need these mental and technical skills, as well as work rate to keep moving and create overlaps. They only need enough speed and power to hold their own in defence.
Otherwise we would see a lot more centres and back rowers in the 6 jersey, like was common 20 years ago when defences weren't as good.