I think I remember the comment. It was said that if you load a long film (220 or 135-36) into the 1501 reel, then the film goes further inside the reel. Since the tank/reel rotates, you can't put in too much chemical, or you run the danger that the excess chemical can be leaked out of the tank. The 2502 may have a capacity for a longer film. So the film may not go that far inside, so less danger of chemical leak.
I do not have a 2502 to compare, so this is why I asked in the first place.