I wish I could give actual tips, but I really wouldn’t know what to say. The best thing to do is probably look at artists you think do it well and try to emulate it on your drawings.
 
Also always remember there is a pony attached to that tum so you really wanna draw their normal body first before attaching a big tum. When it comes to much bigger sizes you might have to bend anatomy just a tad, but there are ways to do that convincingly. Also always think about where the pony’s “chest” ends and where the belly begins. I’ve seen people practically start the belly at the neck and that’s a no no. I’ve done it now and then too without thinking These are pretty general any kind of belly tips but they’re the best I got.
 
Like I said, find artists you like and pics you like and analyze them, it’s probably the best way to figure out how to draw anything to be honest.
 
Sorry I can’t offer more help.