Max is somewhat of a know-it-all, relying mostly on book smarts than actual experience. He is one the youngest characters known to have traveled with Ash. Max also frequently shows tension between him and his older sister May, such as telling embarrassing stories about her to others or arguing with her about the way she does certain things. Max is also shown to have a sweet tooth, as show in the movie "Jirachi: Wish Maker" were he wishes for lots of candy. The Jirachi in the movie did grant his wish, but since Jirachi only teleported the snack-foods from the food stands in the carnival close by, Max had to return the stolen food.
Max to known knowledge has not gained weight in the anime, however he did become bloated after raiding a Pokémon Center refrigerator in his sleep along with Ash in a fanfic by animebellyfan on deviantart.com, which can be found here: http://animebellyfan.deviantart.com/#/d33h8fs Mothman64 has drawn an illustration depicting the events of said fanfic, it can be viewed here: http://mothman64.deviantart.com/#/d3dhrvs