The general misconception is that the bosons in BEC and superfluids are in the same quantum state including the same spatial coordinates. This would result in stacking of each particles' wave function and unlimited reduction in volume of such a substance.

The truth is that the particles in BEC or superfluids do not crowd, but have the capability to do so when perturbed or set into flow motion. They still experience the electromagnetic repulsion between them which prevents their volume from unlimited decrease and this also serves to explain its $0$ viscosity. The internal repulsive EM forces is conservative and does not dissipate energy, rendering no internal friction which in turn renders viscosity at $0$, (there is lack of attractive forces between the particles to increase it's viscosity).