I am a fairly new rookie, myself, Gabriel. Rhizome size can be as much a function of climate as anything else. Look at the Iris Sisters as examples. Most are HUGE. However, as a general assumption, I think, the very small rhizomes with slender, short foliage are SDB/MDB. For me, my SDBs also usually increase faster than most TBs. Can you post a photograph of them? Might help figure it out.
