In my mod I set small radar to have a higher detection chance, and I have confirmed through testing that it works inside the large low-chance radius.
In openxcom with vanilla settings, the large is way better than the small, but if you still have room left in your base, it doesn't hurt to keep the small after you get the large. In the original game, detection chance didn't stack at all except as a lie in the base info screen--which was perhaps a bug.