This one time I was training my Kyurem (who is now level 100) in Mistralton Cave. Then after a wild Pokemon battle... KYUREM'S MAX HP INCREASED! I haven't a clue about what happened. So, a few times, a battle was engaged, and after my Kyurem beat it... POOF. Kyurem's HP maximum went up by 1! Why did it happen?
(This may give you an idea-Kyurem held a Power Weight.)