Smallies are pretty oppotunistic feeders. They eat a lot of aquatic insects. You should not be all that surprised you got one that way.
I agree very much with Dan & what others have said. Smallmouth bass are not often selective like trout may be. I've occasionally caught them on small nymphs while fishing for other species, but when targeting them with nymphs, a larger size is usually a better choice. In my younger days, a size 2 or 4 Montana Nymph was a frequent choice for targeting them.
They're quite fond of Hellgrammites, so any big, dark colored pattern can be very productive in waters where there are a good abundance of Hellgies.
Here's a fly I like for Smallies, even in bigger waters. This particular pattern is intended to imitate large Stoneflies, but again may imitate Hellgies or possibly even crayfish. Really doesn't matter what the bass mistake it for, they'll eat it!
BTW, I generally only tie this pattern now on a size 2 hook. The recipe is on my blog.