I know the answer is probably a simple "it's a stylistic choice", but how come Chopper returns to his tiny hybrid form when he loses consciousness or falls in to the sea? Wouldn't it make more sense for him to return to his reindeer form, since that's his "original/pre-fruit" form? Maybe he sees his little hybrid form as his "true" form now since he was shunned as both a reindeer and a human when he was little? Considering he actually calls his reindeer form "Walk Point" instead of it just being a natural transformation, maybe he really doesn't consider that his true form anymore?
Welcome to Anime&Mange SE @floweringdusk,
Chopper doesn't change form because the "little" form he is in is the one that he is accustomed to the most.
On the other hand, losing consciousness or falling in water doesn't stop the DF powers. In sea water, DF users gradually lose their ability to move and are extremly weak thus they are called "a hammer" in regards to swimming.
If you remember, after the fight between Luffy and Rob Lucci (think chapter 427) when Lucci is knocked out but still regains his leopard form regardless.