Dude... it happens. And I can't tell ya why.
It's happening to me. I weighed myself the other day. I weigh in at a grand total of 102 pounds. With shoes on. And wet hair. lol. But I'm getting older... and somehow those 102 pounds are distributed differently from my 98 pounds in college. On the down side, my face looks fatter. On the upside, I can finally fill out my tops. 🙂 Getting "puffy" where you used to be gangly is just part of getting older.
As for James Marsters, well... I'm sure he was in the gym like every day to get that season 6 body. But even that does not explain the skinnyness and general angularity of his physique. Seldom have I seen an adult be able to maintain that kind of form (much to my dismay, given that my hubby used to have that form). I'm more like Nicholas Brendon, I guess (but, you know, a chick)... I started out naturally skinny and now I'm still roughly the same weight, but, somehow, not quite so angular. Seems natural to me.
Besides, so what if he did gain weight? Like Mesektet Ra said, he was still Xander!