When an actress who is a series regular becomes pregnant the showrunners have a limited number of options: write her character out temporarily on some pretext, make the character pregnant, try to hide it, or make the character fat as happens here.
Jill Kendall (Jaime Pressly) is a a wealthy, self-involved divorcee; repeatedly in and out of rehab, Jill relies on the support of her friend and sponsor, Christy
The pretext for Jill becoming fat is that her addiction has developed into comfort eating as she tries to work through some emotiomal personal issues.
Jill Cady was the makeup artist.
I don’t know who was responsible for the fat makeup but it may well have been Clinton Wayne (Wayne Studios) who was responsible for the previous fat makeup on the show.