Fat cat...litter box problems....

My aunt has got her female cat, Catie, a few years ago. When she rescued her, the poor girl was abandonned and pregnant. Well, I'm not quite sure what happenned, but they had to get her aborted...wich is sad already, then she was nutered (spayed?) but since then she has been gaining pound after pound, even on the diet prescribed by the vet. She is now quite overweigh even if she eat very little dry diet food.
She had never had any problem going in the litter box, even if she's fat, the only problem is poo would stick to her long-haired bottom and my aunt has to wipe it up for her everytime cause she has some trouble cleaning herself there, even when she gets a trim.
Now...she is 6 years old, wich is not that old for a kitty, right? Well, the vet has seen no medical problem with her, but she poos quite often on the ground, not too far from her litter box. She does pee in there, so she has no problem to get to it, but half of the time, the poo ends up on the floor (and it's not diarrhea, so I don't think it's a get there in time problem..she has very hard little stools), and my uncle can't stand it anymore and says he wants to have her put to sleep each time it happens, wich breaks my aunt's heart...
Any advice?

Do you know what size litterbox your aunt has? A larger box might help.

Also, I haven't had a girl kitty, but I know that my boys often pee in one spot and then end up "poo"-ing in another spot. If the box isn't big enough, she may just end up with her bottom outside of it. Has your aunt watched her to see what she's doing when it ends up outside the box?

If there's no way to get her to stop going outside the box, she might be able to get some sort of mat to protect the floor. I hope your uncle isn't serious about putting her to sleep. Poor baby!

Unfortunately, he is! Because he is sick and tired of cleaning up (of course, that wouldn't be a good reason to any of us...but who knows what he thinks..)
The thing is, it couldn't be that her butt is hanging out, because it is too far from the litter to be possible. It also happened a few times in other rooms...

Do you know how often she is changing the litter? Is it clean enough for the cat's liking?

Also, is the cat an only cat? Is there a possibility that another cat is intimidating her while she's trying to use the litter, making her hurry or go in other places?

Hmm..

No...she is an only-cat!;P
And the box is cleaned everytime she poops or pees in it...the vets told them about that, and they are sure to always leave it clean!

Hmm... this is a tough one.

Maybe she needs a hair cut? You said her hair is long and that the litter (and other things) get stuck? Maybe she doesn't like the feeling, so she avoids the litter?

(I'm grasping at straws now.)

Thanks! You're very nice! I appreciate!!lol
Unfortunately....lol...her hair is always kept at it's shortest, especially in the butt area...

Try providing two litterboxes. The cat might be wanting to do different things in different places (so to speak). Nano uses one box to urinate and a different litterbox to defecate.