Point of Order. Velma was not fat. She was actually quite normal-looking, but she wore a thick sweater that was bunched on her arms and had a thick turtleneck.
https://en.wikipedia.org/wiki/Velma_Dinkley
It made her appear to be bigger than she actually was. At least as she was portrayed in the original series. I don't know about more recent iterations.
Velma in the movie was hot