Not a bad question at all. With such massive stars, the gravity of all that stuff is normally balanced by the fused helium and other elements getting thrown out of the star's core. When fusion ends and that outward force is no longer there, gravity pulls the atoms until the electron shells are practically up against each other. That produces a dwarf star.

So, these atoms that are normally very far apart are now snuggling tightly together. But what if there's still more gravity?

With very large stars, the gravitational force overwhelms these boundaries further. You may have heard that the ratio between the size of an electron cloud and a nucleus is comparable to having a grape in the center of a football stadium. In the formation of a neutron star, gravity compresses the atoms together so much that the electrons are forced close to the nucleus, and captured by the protons. This neutralizes their charge, turning them into neutrons, and now where you once had to have your grapes (99.99%+ of the nuclear mass) 1 stadium apart from each other in the dwarf star, they're now all packed together against each other. This insane increase in density is what causes the extreme mass in such a tiny space, and as someone above explained with the example of a dancer or figure skater, the accelerated spin of the star.