Young adult book with teens exiled on prison planet

Partial match, depending on how clear your memories are, would be Exiles of Colsec by Douglas Hill. The basic premise - group of teens exiled to a remote planet - matches.

Probably written in the 80s.

1984

First of a series.

A trilogy - sequels were Caves of Klydor and Colsec Rebellion.

There were 3 or 4 teens who were the main characters, I believe.

5 - Cord, Jeko, Rontal, Heleth, Samella

One was a muscular boy that looked fat. He uses a kris blade knife.

Partial - Cord was described as muscular but short, a combination that made him look far to others. Used a club I believe rather than a knife.

There were large cat predators on the planet. I think the protagonist befriends one of the cats who becomes his companion.

Not a clear match. There were giant worms, and also aliens who had some cat-like features (fur, sharp teeth, sounded like cats). The latter managed a degree of telepathic communication with Samella, but didn't become companions of any of them.