Well, in Colossus' defense, his fights against classic Juggernaut, he always just duked it out with him. If he put any amount of skill into those fights, there's a good chance he could have won, in one sense or another (that doesn't necessarily mean a Knock Out). However, today, Colossus is one of the dominating Power-Houses in Marvels roster. He could put a serious hurting on Juggernaut (Not saying he'd totally own him, but he'd win today).
I'm also curious, did Colossus ever fight Juggernaut again after that upgrade or whatnot he received from Magneto? If not, then I believe there's room for speculation on Peter taking Marko down.