A TV series set in a country where 30% of the population is obese and 1.5% are morbidly obese, may signify that Hollywood is pushing things in the right direction. Still, Buffy looked better in Season 1 than in Season 7.
Yeah, but who wants to see a pot bellied vampire?
As I said before, whether by intent or by accident, a thinner Buffy in Season 7 would make sense, considering the life style that she would have to live as the Slayer. No sitting at home at night watching TV and eating popcorn, probably, in fact, more meals missed than eaten.
Different character and different universe, but Kelly Armstrong's werewolves in "Bitten," and its two sequels have to eat huge amounts of food just to support their body metabolism. It wouldn't be too much of a stretch of the imagination to say that the same would apply to Whedon's Slayers. That energy has to be fueled somehow, and it would fit in pretty well with Faith's statement that Slaying always made her "Hungry and horny."
Sarah's most noticeable weight loss was in Season 7, which would fit in with Buffy's having to feed a houseful of Potentials. By the time that Buffy got home from patrolling, most of the food would be gone, so, no supper for the Slayer.
It could have been by intent, and it just never got mentioned in the story line, or just by coincidence that Sarah lost the weight, and maybe no one agrees with me (I know at least one here who probably won't) but it fits in quite well with how I think that it would have actually been.