"At least 50 pounds"? According to their listings, the biggest difference between them was 28 pounds (248 to 220). That was when they were young. Then it was 7 pounds (260 to 253). Now it's 2 pounds (255 to 253). Garnett is also really 7-1 (it's been said many times), not 6-11, like Duncan.
Because Duncan does center-like things, people act as if he's 7-1, 275. But in reality, he's smaller than Garnett and Nowitzki. He is the strongest of the three, but the point is, he played bigger than he actually was for years. And because he was so ridiculously dominant, this organization took him for granted and didn't get him proper help as he aged. Now we're seeing the effects of him having to carry far too heavy a load for too long. He played hard, grinding minutes, many of which came in the playoffs, which are harder minutes to begin with, for various reasons.
Nowadays, it oftentimes makes sense for Duncan to guard centers (not against true centers with skill, though), because of his declining mobility, but he's really a four and a half. The difference between him and guys like Garnett, Nowitzki and Gasol is, their teams never asked those guys to act as full time centers. In the case of Garnett and Nowitzki, they were never asked to act as even part time centers. They played far easier minutes. That's why Nowitzki in particular hasn't declined. Nothing he ever did lends itself to physical deterioration.
Even with a far heavier load and more hard miles on him, Duncan has drastically outperformed Garnett since he became a Celtic. Even this season, with Garnett supposedly bouncing back and Duncan having his worst season ever, he was still slightly better than him. The only two things Garnett has on Duncan are mid range shooting and face up defense.