A few things:
A moderately decent butt expansion animation I found on DeviantArt:
link
Two sequences I recently commissioned that I uploaded to my DeviantArt gallery include some awesome butt expansion.
MAYA'S BORDERS:
Page 1:
link
Page 2:
link
Page 3:
link
Page 4:
link
Page 5:
link
MIRANDA
Page 1:
link
Page 2:
link
Page 3:
link
Page 4:
link
There's a couple of other comic sequences that feature some butt expansion but mostly focus on growth in my gallery.