A mealworm looks very much like caddis larvae which you will find in pretty much all fresh bodies of water in our area. Therefore, it's a very natural looking bait. Maggots on the other hand are, well, housefly larvae and grow in rotting flesh vs. underwater.
Maggots works well when you use them to "tip" a lure that already has some visual appeal. I find meal worms work better on a bare bait hook.
Here are some pics of a caddis larva. I firmly believe this is why they work so well and that size isn't the issue.