Well you see, in a last-ditch effort to save the ring, Sauron used dark magic to teleport it to another dimension (Star Wars) as it was being destroyed. Little did he know, Gollum was teleported with it because he was with it at the time. Gollum woke up on Tattoine right as ep. 6 was ending. He quickly discovered he was strong with the Force. He said "Screw the Ring, it sucks anyways," and spent the next 26 years mastering the Dark Side of the Force. After that time, he started the First Order and set in motion the events that led to ep. 7. Oh, and he hit a growth spurt. That's why he's a freaking giant.

So the fact he's massive is actually normal for a six-seven hundred year old hobbit and not overcompensation? Huh, learn something new everyday