There were several inflations in the Oz series.
Let me see... The Tin Woodman of Oz had a tribe of balloon people. Scalawagons in Oz had a magical oil that could make people inflate. Whichever book first introduced Jenny Jump (The Wonder City of Oz?) included self-inflating pants. The Cowardly Lion got turned into a balloon in, Ozoplaning with the Wizard of Oz, I think the title was? One of the Ruth Thompson Oz books. There was a second race of balloon people around, living high up in the sky, but I don't recall at all which book featured one of them.
Another one of Baum's books, The Magical Monarch of Mo, had one inflation-themed chapter, after one of Mo's princes went through a giant's clothes-wringer and needed help regaining his proper shape.
I did a bit of searching, and it actually sounds intriguing.
http://members.cox.net/cruentidei/oz/review/htp/htp_11.html
Apprently, its illustrated as well. The book its in is abou $25 though..