Why wasn't he a Bigger Star?

He never care for it. Lost interest for it. In 2010 he wanted to live and work in the country of Europe. He`s there now.

I've heard rumors that he's enormous.

In the 80s he starred in some very good to great movies that he was excellent in (certainly not comprehensive, just what I've rated):

My Bodyguard (1980)
The Outsiders (1983)
Rumble Fish (1983)
Drugstore Cowboy (1989)
There's Something About Mary (1998) - imo, he stole the show with a supporting role

He was still a household name at that point, but though he's continued to be a working actor since then, he wasn't on my radar much after that, except for a serial killer movie The House That Jack Built (2018) that got a lot of hype in horror circles.

I assume TheHardSell nailed it, except for the Europe part; he likely chose to focus on interests other than stardom and makes enough money acting to support them.

He's one of my favourite actors. I guess he never wanted to be a big star. But he's been in some great movies. Some of my favourites:

Factotum (one of my favourite movies ever)
The House That Jack Built
Beautiful Girls
Drugstore Cowboy

He is what he is, a solid actor who brings it when he's cast. I'm sure he's more than happy with his path.

I honestly thought he went farther than his talent should have taken him.

He’s superb in Wild Things.