@redbedheadd
Not sure what you want us to say or do. I mean of course YANBU to have been a bit annoyed that a fat person was squashing you a bit. However, it's a bit late to moan now.
I suspect the women did not even realise how fat she was tbh.
I went to see a show last year, paid £70, and the 2 women next to me kept fucking yabbering and jabbering on, saying the dialogue 2-3 seconds before the actor on stage, and saying 'I love this bit, I like this bit, this bit is good, this bit is weird, ha ha ha.' 
I wanted to fucking kill them. 25-30 minutes in, I turned around and said with a pleading tone ... 'Will you PLEASE be quiet. You are spoiling the show for me and my friend.' They ignored me, but stopped immediately. However, about 10 minutes later they started yapping again. 
I would rather have been sitting next to a very large person who was fucking quiet to be honest. Even if I WAS a bit 'squashed!'