It does need to be remembered that Gellar is quite small. The height usually quoted on fan sites is 5' 3" and I would hazard a guess that she might even be an inch or two shorter than this. The "ideal" weight for a 5' 3" famale is anything between 105 and 130 pounds (as a very generalised rule of thumb), depending on all sorts of different factors. I would guess that Gellar is probably around 98-105 pounds, which does not really lead to any serious cause for concerned.
Don't forget that she is extremely active and spends a lot of time in the gym - and probably did so with great dedication when she was making 'Buffy'. It's very likely that she has a carefully controlled diet. Let's face it, were she to suddenly put on a few pounds she would be accused of being fat. It happens now. I have seen examples on posting boards of this very accusation in response to pictures taken at odd angles, etc. Either that or ridiculous rumours are started suggesting that she is pregnant!
There certainly is evidence to suggest some weight loss between the third and fourth seasons of 'Buffy' and it has been said that she was working herself too hard around the time of the final season, with a few concerns expressed that she would make herself ill if she didn't take time out. Having said all of that, look at pictures of her from the time of 'All My Children' or 'Beverly Hills Family Robinson' and it becomes clear that she has always been very petite.
As others have already mentioned here, she lost some "baby fat", but it probably wasn't anything more than that. Personally, I think she looks very healthy.