McGwire said he took roids to help him recover more quickly from injuries. (Wheaties may or may not do that as well).
You seem to acknowledge that steroids make an athlete "bigger," meaning more muscular, presumably -- more powerful. When this additional muscle power ("bigger," as you say) is combined with the God-given hand-eye coordination necessary to hit a baseball or throw a baseball, one could reasonably assume that the athlete who takes roids has an edge over the athlete who doesn't.
Roids won't make you see the ball better, or give you the ability to make contact with the ball more easily, but they do make you bigger, as you acknowledge, thus affecting how far the ball might travel once you do make contact with it.
Advantage roids user.
But, even if you believe that roids only help an athlete recover more quickly from injuries (as seems to be the justification they all give for taking them), and provide no additional power, that still gives these athletes an unfair advantage over those who didn't take roids.
Bottom line: If roids didn't provide some advantage, why would some athletes take them in the first place? You seem to be arguing that roids are no different than a placebo. That is clearly not the case.