QM is mostly a mystery to me, but my understanding is that QM allows only discrete, and non-zero, 'sizes'. Hence for example an electron cannot appear halfway between two orbitals. Relativity on the other hand does not have this limitation and thus the unlikely possibility of a singularity. At the size of a singularity, Relativity is in conflict with QM. Hence the need for a Theory of Quantum Gravity in order to understand black holes.
Hope someone corrects any misunderstanding I might have expressed here.