Bet that small one is dense af!

Nah, they chopped that one down 96 years ago.

Looks like you and the tree have something in common

Hah gotem

My trunk do be thick.

I recently removed some 2X4’s from a house built in the early 80’s and it was ridiculously dense with legit too many rings to count. Nowadays there’s like three rings in them

One is the base of the trunk, the other the end of a branch.

Bonsai?

Altitude is the my guess as to the difference, assuming they are even the same kind of tree

no, trees can stay very small for a long time if the conditions are not right
for example if the available light is too little to sustain a bigger tree, if one of the bigger trees dies and more light and nutrients reach the small tree, they will then start to grow bigger
that's how people make bonsai trees they meticulously create the conditions for the tree to be able to sustain itself and live but not too much so they stay small, there are decades old bonsai trees

It’s altitude plus lack of water.

This picture has me in it and I don't like it.

There are hundreds of years old bonsai trees.