Yep, it happens almost every season because of new animation possibilities etc. and in general advancing technology. It will always happen at the start of a new season because well, it would be weird for Ash to suddenly change in a new episode, wouldn't it? And yes, Pikachu got thinner to match the better Sugimori proportions, I think. And yep, Ash should stay 10 so they can't afford to make him older, as they almost did. It wouldn't seem right for a big guy with a beard travelling on a journey with his Pikachu to become the "Pokémon Master", will it? lol