It all depends, but I've never had a problem with "fat" bettas. Most likely she is swelling with eggs (Not very common unless a male is present O_o) Here are some photos of my girls I used to breed. They do tend to have round little tummies. Could you post a photo if she looks anything other then these photos?
Thank

I used to breed Betta, so feel free to ask me questions!
Female to the right

The two to the right are Females. See their fat tummies?


In the end, I don't think you should be concerned, especially if her behavior hasn't changed in any way. And if she develops 'Zebra Stripes" along her body, don't freak out. Its just her way of showing she is ready to spawn (which she shouldn't naturally do without the "sight" stimulation of a male lol)
QUOTE(Hobbity1 @ Mar 25 2007, 01:22 PM) [snapback]648936[/snapback]
i noticed this morning my new betta is a bit chubby, ive only had ehr a few days and she isnt constipated as i see lots of nice poopies in her tank(i did a full tak change yesterday) she isnt showing signs of being sick or anythign like that, its just like she gained a little weight, what can i do? do i need to fast her? she isnt constipated, as ive mentioned , shes still pooping, but are females just differently shaped then males? ive never had a female before. she looks kind of busty or barrel chested, lol. just a little more round them a male betta should be.