Ellen is not fat.
I'll be the first to admit, my grasp of anatomy isn't the greatest - not to mention the inconsistency inherent in literally everything I draw - but I do try to give my characters distinctive body types. I could show you neck-down silhouettes, and you'd still be able to identify who's who.
Ellen has a broad bone structure, and her upper body is actually quite muscular, but she doesn't have any definition to her muscles because she's got a nice layer of zaftig girltissue over it - tits, belly, hips, thighs and butt, none of which have yet succumbed to gravity and time.
She's heavier than Nicole or Gina, certainly, but she doesn't have Lily's shapeless dumpiness. Ellen is curvy, she's voluptuous, she's rubenesque, but she's not fat.
I wouldn't say she's a bitch either, although the term "bitch" is perhaps one of the most broadly defined of the major swear words. In this instance, it's clearly being used to refer to a person who is unjustifiably or overly moody, aggressive, and whiny.
I don't think this description fits Ellen. She has, throughout Leftover Soup, acted as a sort of foil (a "straight man", if you will) for Jamie and Max's odd behaviour, and I think she handles herself pretty well, all things considered.