Ash's main goal in life, as described in the first episode of the anime, is to be the world's greatest Pokémon Master. Ash began his journey at the age of 10 as a warm-hearted and well-intentioned but stubborn, rash and amateurish Pokémon Trainer. He was forced to accept the cheeky Pikachu from Professor Oak as his starter Pokémon because he carelessly broke his alarm clock in his sleep and woke up late on the day he was set to get his first Pokémon. He was determined to achieve his goal, however, and when Pikachu saw that he was willing to compromise his own safety to keep it safe he formed a strong bond of friendship with Pikachu.
Ash is shown to (at times) have a large appetite, usually when he is prompted to eat all he wants (typically shown in the Diamond and Pearl series). He is also shown to be competitive with his eating in the episode AG118 "Less is Morrison", when he competes with an overweight boy named Morrison, to see who can eat more. Unfortunately the outcome was not shown.
So far, Ash has been known to gain weight four times in the anime. Each one being very brief.
At the beginning of DP055 "Throwing the Track Switch" Ash was shown to be already bloated after eating too much and his stomach was being massaged by Brock's Happiny.
Ash also (briefly) gains weight in DP104 "Sleepless In Pre-Battle" when the Officer Jenny of Canalave City thanks Ash and co. for chasing-off Team Rocket.
In DP155 "Double-Time Battle Training" after feasting, Ash had a full belly shown in the scene in their hotel room (when Zoey asks where Dawn is).
Ash's most distinct weight gain so far was in BW040 "Cilan Versus Trip, Ash Versus Georgia!" in which his bare stomach is shown after eating a large dinner in friendly competition with Stephan.
Ash also became bloated along with Max after raiding a Pokemon Center's refrigerator in his sleep in a fanfic by animebellyfan on deviantart.com, which can be read here: http://animebellyfan.deviantart.com/#/d33h8fs
A picture based on the fanfic was drawn by mothman64, another member of deviantart.com, the image can be seen here: http://mothman64.deviantart.com/#/d3dhrvs