Part 1 | Info on Ursula growing into a giant in Disney's The Little Mermaid?

I think the thing is that with most Disney movies, the villain will grow large or shape-shift to be very menacing to portray the purest of evil. Remember Jafar in Aladin? He grew into a giant cobra just to toy with Al instead of just finishing him off instantly. Same with the villain in Sleeping Beauty. Disney wanted to use the idea of the main villain's ego and under-estimating of the under-dog hero to eventually do themselves in. As for the reasons why Ursula was offed so easily, it was probably done to lessen the amount of violence levels and amp up the romance factor in the movie.