Why don't physicists consider a change in density a change in the gravitational field?

i don't know about physicists, but i understand that if i take a single particle of matter it will have a certain effect on space. this effect is called gravity. if i take that same particle and squeeze it until the particle is 50 percent of its original size it still has the same "weight" thus the same effect on space. i could be wrong but my following example and understanding comes from the show the universe on the history channel(i'm addicted); so if i am wrong please correct me. if i place a rock on a bed and it sinks in one inch, the same rock compressed to a size 50 percent of the original will sink in the same amount. i realize this is probably a terrible example considering physics is far more complex than rocks and beds but i honestly don't know enough to explain it in another way. not that i don't desire to know physics completely, i just find myself distracted by my other desires. if only i had some sort of structuring to enforce learning.