Claremont is partly to blame. Whenever he wanted to show that a bad guy was a real threat he had him or her knock out poor Colossus. He was always written as ”one of the strongest persons on earth”, but he rarely got to show it.
I remember when he returned after his injury in the mutant massacre. He was bigger and stronger than ever. But then they seemed to forget that.
He needs more feats. I loved how he was Gladiator’s equal back in the Dark Phoenix saga.
I remember and issue of ... New Mutants? He was training the next generation of strong guys like Rockslide. They didn’t stand a chance. A really good scene.