I'm fairly sure they're implants. The reason I say this is that I distinctly remember her appearing in an episode of Baywatch long before she started playing Cordelia on "Buffy". She played Hobie's girl in the episode when hobie threw a party and someone stole his dad's hang glider. She was slender and had long straight hair parted in the center and her character was supposed to be about 15-17. She was no more than a B cup, and a small one at that.
I checked her out at IMDB -
http://us.imdb.com/Name?Carpenter,+Charisma. She was born in 1970 and that episode played in 1994, so she had to be 24 at the time it was shot!
While her body has undergone many changes since that episode, thanks to a good trainer, there is no way she could have grown to her current size and shape without implants.
I'll try to find a picture from that Baywatch show for you to judge for yourself.
BTW - IMDB also confirms that she was a cheerleader for the San Diego Chargers in 1991.