Thinning out large trees, so smaller trees can grow, is not a game mechanic. Small trees do not grow larger in this game. I have experimented numerous time, removing large trees, and leaving smaller ones, then marking the location on my map. I have never seen a tree that I did not plant, grow beyond the size that it was when I first saw it. There may be mods, that influence the growth of flora in this game, but without a mod trees do not grow or respawn.