A small UV that is "finely tuned" still nets the same results. By finely tuned I mean slow enough flow running through a bulb that is changed out every 6 months. And since neither small or large will ever accomplish 100% eradication, the best you can hope for with a UV is the overall number of nasties stay down to a manageable level.
In a weird sort of way, running a UV somewhat replicates nature - since parasites are always present in the ocean, but usually at manageable levels for the fish to deal with. But even still, the ocean contains a gazillion gallons of water diluting those diseases from the fish. A healthy immune system can keep up with that. No "glass box" - no matter how large or how many parasite zapping apparatuses - can afford fish the same level of protection that mother nature provides. Which IMO is why it is imperative to QT; to avoid introducing any rapid spreading diseases into your tiny glass box.