You could see Ann Wilson really starting to go in the 80s. She wasn't even fat yet (but clearly hiding something with all that black) in this video, but they just focused on her sister instead.
I watched a Heart
Behind the Music episode last night, and they basically admitted that's what was going on. The record company was also pressuring Nancy Wilson to confront Ann about it, but she refused to do it. And why would she when she was getting featured more? By the late 80's Nancy was already a cow.
And I love how fat chicks think wearing black helps.
Also according to the BtM episode, Nancy has not been in a long term relationship since the late 70's.