well, i haven't done any math either :) but suppose that you had a piece of paper standing upright on the moon. so when you looked down on it, you could only see a very thin line. now, the chance of it getting hit from that angle is obviously very slim, and is directly proportional to the surface area it occupies.

however, sideways, supposing it is possible, the chances of it being hit increase because the size increases as well. this isn't a particularly good example, because a piece of paper really lacks measurable volume, but it's the same idea - if you had, say, a block that was 1" thick, 300 miles wide and 800 miles tall... the chances of it getting hit by a meteor sideways are much higher. correct, though, in that the chances of this getting hit are proportional to the surface area of the object.

so basically, i think we're both right :) we just both weren't super clear in our explanations.