Actually, they are equal. The low-power machines aren't "racing" against the high power machines. For any given difficulty level, it doesn't even matter how many other machines are generating. If all the other machines dropped off the network, it wouldn't increase the number of blocks you generate (until the next difficulty level adjustment).

The only thing that counts significantly is the difficulty level and your khash/s. There are some relatively minor effects due to network latency, but they're not a big deal.