Again, genetics also plays a big role. Merrin is another good example. Even when he was sporting a muffin top, he could still play 80 minutes of footy in the forwards, unlike plenty of other forwards who would appear much leaner. Clearly his fitness is incredible, but take off his shirt and he’d look big but not defined.
On the gaining of size , once you have been training hard with weights for a few years, as even the youngest players have, it becomes harder and harder to gain size (absent taking steroids).
Losing a few kilos of lean muscle is easy - simply keep your exercise regime the same but eat less. It is also quite easy to gain it back once you have been big (various theories abound about muscle memory and stretched fascia etc).