If I'm not mistaken, objects as small as Enceladus shouldn't manage to be round on account of their own mass and gravity well. In their forming stages, objects this size cooled and hardened quickly into irregular, potato shapes.

When an object becomes spherical under its' own weight, it's called Hydrostatic Equilibrium, and this moon is simply too small and lightweight to get there on its' own. But because of the humongous tidal tug-and-pull it is subjected to by Saturn in front and Titan behind, the small core has been kept like a volcanic putty for billions of years, enough time to slowly but surely get there.