well gravity can change the size of things, ie some red giants become neutron stars, a fraction of their original size but very dense. Soime stars become black holes which have a singularity, ie size=0!
I was destined to grow to well over 6' like most of my male family members, but my auntie kept patting me on the head saying what a good lad I was so I only grew to 5'9".
If you imagine that most things are full of gaps between the atoms, you simply need to compress the atoms a bit closer together. OK, that does pose it's own problems depending on the nature of the atoms, but usually it's possible.
Big problem is when you release the force it goes back usually bigger than before which is bad in some cases.