>>149988747
The contrast between Kellman's art showing Gloria's ass in picrel confirms it. But I don't think we could have gotten Gloria with her previous larger ass because it would've constantly pushed around and bounced against the other characters constantly. I don't know if I heard about her giant butt being a challenge for the animators in either an interview I saw in a short or the behind the scenes material included on one of the DVDs, but it does imply they must have had a model with the bigger butt at some point during production.
>>149988746
I don't know if I can dig up any concept art, but the maquette's got the ass.