I don't think Harvey looked overly big in the show. I think Edna should be a very large woman. The whole point of the show is acceptance, with weight being a primary topic. Edna even sings about it, "I can't stop eating/Your hairline's receding..." etc. Something tells me a pleasantly plump woman would not say she can't stop eating...
I personally think it was a vanity issue. Travolta didn't want Edna to look too fat. I just think he was awful for the part. The character, the voice (which was AWFUL), the whole package just didn't work.
I hold a degree in Musical Theatre from Montclair State University. It is useless. Now I'm funny for money. Oh, and I sing.