They can.

I use Magnums or Over size decoys pretty much exclusively. I hunt marshes, tidal creeks and beaver swamps primarily. I would list it as small water bordering bigger water (river is about 500 yards wide). I have killed ducks over the mags, and over the older smaller blocks I used years ago.

I don't think that mags hurt you, but in small water situations, they probably don't provide the advantage they do in more open situations. In tight water, I fill holes I DONT want the birds to land in with decoys. Bigger blocks can occupy more space sometimes and that can be helpful. Its largely subjective. Just my experience. If you are in the right spot, hide yourself well, don't overcall.... you'l kill ducks over painted bleech bottles. Thats not an exaggeration either.