There are two mistaken assumptions here:
1 - When you pin a team in their corner it usually isn't the bigger forwards doing the early work coming out of trouble. Primarily it's the back 5, maybe with help from a back rower. The middles don't usually get involved until late in the count.
2 - There is no evidence to suggest that smaller forwards are more effective at the back end of games than at the start. If that was actually true, the majority of NRL coaches would bring their smaller guys off the bench so as to avoid the "ineffective" period early on when the opposition is fresh. If anything the opposite is true. Smaller guys play a similar role across the 80, but we see coaches making well-timed subs of big boppers for impact.
The way to tire out opposition forwards is to hold the ball, win the ruck and to ask questions everywhere on the field and in different ways. When you do that, it takes the sting out of those play 4 or 5 hit-ups and you start winning the field position battle too. Pinning teams in their end is part of the picture and it can certainly have a mental/momentum effect - but it doesn't wear out big forwards. They just walk back and rest for a few tackles while the back 5 do the hard yards.