I'm afraid you are confusing mass and volume. Try to think of it this way. What weighs more, a ton of brick or a ton of feather?
Well, the answer is they both weigh the same. But a ton of brick would occupy a lot less volume than a ton of feather, uncompacted of course.
When we have lots and lots of matter clumped together, the overall gravity of this thing increases. This would cause the object to shrink in size due to gravity. But the overall mass remains the same.
What would keep such an object from collapsing further is the outward pressure by the mass itself. So, eventually such an object would settle on a certain size. If you add more matter to it, the gravity would increase and, depending on the outward pressure, the object would shrink in size again while its mass would increase.
In physics, there is a principle called the pauli exclusion principle. Look it up. This principle sets a certain limit to the collapse of matter due to the outward pressure of subatomic particles. We call these objects neutron stars.
But what if the object in question has more mass, thus more gravitational pull, to overcome the pauli exclusion principle?
And this is where my question comes in for you. Can you explain to us what force or principle would prevent such an object to collapse further?
Edited by Taz, : No reason given.