Awesome Possum! Increasing the mass, as well as decreasing the radius, both lead to an increase in the escape velocity of the body.
In other words, as you start cramming more and more material into a smaller and smaller-sized object, the object's escape velocity increases. That is, it becomes more and more difficult for an object to escape the object.
In Physics terms, the greater the density of a spherical object, the more its escape velocity will be.