....ramming the huge materia into Meteor? The only explanation is Cloud saying they need to study the huge materia to defeat Sephiroth.
But even with that, the huge materia "missions" start when Cloud is not with the party. They listen to Shinra's plans to destroy Meteor and defeat Sephiroth, then it was basically "we must stop them from attempting to destroy meteor!"
FF7 is, was, and likely always will be my favorite game ever, but I can't think of any reasonable explanation for this.