In Alpha Sapphire and Omega Ruby, there is an Ace Trainer found in Rustboro who will give you a Float Stine and proceed to tell you who it will halve the weight of a Pokemon. If you go upstairs and then back down, you will find a fat hiker in the place where the Ace Trainer used to be. If you talk to him, he'll say, "For some reason, I put on a lot of weight recently."