How do you talk to your manager when you find out a co-worker with same duties on the same team makes more than you and is performing at a lower level?

I just found out a coworker on the same technical sales team as me is making about 25% more while I performed 3X as much. Technical sales typically has more of a base than commission such as 70/30 split compared to a 50/50 split for typical sales people, I am referring to his base, which also pumps up his variable. How do I start the discussion without sounding like I’m whining and singling out this one coworker? Also, another coworker, who was #2 behind me makes about 25% less than me.