Mine did that for about a year. Got bigger than a basketball, I'm thinking 20" beachball! I set up a new tank and moved it into that system and it split into three. Two of the splits were starting to split again but I moved them to a lfs. One sold and split in the new tank, the other I brought back home and put it back into the old tank and it has been wandering there used to stay put) and both splits are growing (but not splitting).
I think I heard a story that someone dipped their nem in fresh water and that caused it to split? Don't do that (unless someone else chimes in to verify that rumor). Other people will cut them in half, and may have other methods to stress them into splitting if you want to scale it back? The lfs made some dough from my split, and I am glad that the two halves are smaller than a beachball.