How to increase terrain definition ?

Hello everyone !

This is very basic, but I can't find a way : I have a 2K heightmap that I imported as a 513x513px texture for my terrain. I have worked a bit on it, adding details and such. Now I would like to get a more defined terrain, because right now in some areas, the tris are really visible. I tried changing the heightmap import quality, but it doesn't seem to change anything, apart from saying "Require resampling on change". If I change it to something lower it does get a lower polycount, but higher it stays the same. Am I on the wrong setting ? Am I missing something ?

Thanks a lot