I was watching "From Star Wars to Jedi: The Making of a Saga" and "Classic Creatures - Return of the Jedi"and there was a rather extensive segment about Jabba the Hutt and what went into designing him. Phil Tippett showed a series of different maquettes that showed the evolution of the Jabba design and he relayed Lucas' comments as the design process went along (i.e. "too human", "too alien")

It's obvious that Lucas was not happy with the Jabba scene as it played out in 1977. Jabba just didn't come off right and the scene was redundant. Lucas took the opportunity in ROTJ to push the Star Wars universe by making Jabba a big alien looking slug, and that was definitely more appropriate.

There was absolutely nothing in the Lucas interviews or anyone else's comments that the Jabba in the first film was supposed to be a slug. Not even Lucas' comments in the documentaries touch on that.