I'd say by looking over all of the game after you've beaten it, it feels kinda short, but I don't know why, considering how much replay-ability it has. Including the Special Worlds, it has 16 worlds in all, each with about 5-7 levels per world. Each level has 3 star medals to find, and if you want to get the special level, you have to beat all of the levels with Mario and Luigi, and get the top of the flagpole in each.
But this is a considerably easy game, which may make it seem short.