I've made an extremely simple scene, as a pretty minimal example to reproduce this issue. I'd like to know what I was supposed to do differently to get reasonable results. To make this scene:
- Make a plane.
- Extrude one edge up to make a wall.
- Knife a little cave entrance in the wall, touching the bottom edge of the wall.
- Extrude this cave in a little.
I want to UV unwrap this structure, and I did so with the following seams (the selected edge is also a marked seam, by the way), followed by Unwrap:
From these marked seams it's incredibly clear to a human what the four islands should be. From this image it's clear that Blender messed up the complicated concave face; that's not even really my problem, I'm okay with splitting up that face with more edges (although I'd appreciate tips on making unwrap go less crazy with complex planar ngons).
The real issue is that the selected edge gets mapped down to being incredibly small for the large square ngon floor face. To see how bad it is, observe when a color grid is overlayed:
It's easy to fix manually in this case, but what am I doing wrong in general?
The .blend file is here.