Hi Fifi - there are a variety of methods.
On page 3 of this post
http://www.process-productions.com/f...t=35977&page=3 i a post by Airwolf a user who once made the props used by Bambi Blaze and now Taylormadeclips.
Here is a Deviantart User who had some amazing results with their own rig (sample images on their deviantart and videos have been on youtube)
http://thesweaterpupbreeder.devianta...-Nah-219384131 the journal entry I've posted was a description of their rig.
Maxgrowth Productions (Found on here, youtube, taylormade clips) might be willing to have a chat about it too as they advocate for more people contributing to the fetish/genre
And
https://www.youtube.com/user/mondovideotron was an attempt at making a mainstream weightgain series (featuring expansion of body parts through rigs and costuming)
I myself prefer a very balloon/inflated look.
I hope this helps
