Besides the fact that balloons rarely grow to the full 8", usualy reaching about 6" maximum, they also have a distinctly 'deep' body, which is where they get the 'balloon' name from and are shorter, wider and deeper over-all. It is a similar mutation to that of 'balloon mollies' - if you would like to look them up and compare those to ordinary mollies you'll probably be able to see the difference more clearly than you can with the gouramies.
