UV unwrapping doesn't use full UV Space, but roughly a quarter of it This problem occurs rather often the last weeks to me. I'm using Blender 2.83.0 (can't move to a newer version because of a few add-ons I need to use frequently). But nevertheless importing the project for testing purposes into 2.90, the problem stays the same.
If I unwrap my model, Blender barely uses the full size of the UV space. I didn't find any solution for this problem.

Even if I use the UV Packmaster 2 Add-On, the UV Space doesn't look any better:
Blender generally scales the UV relative to the longest (fully self-connected) piece. To make it take the full space, add seams to the longest piece(s), to make it roughly the same &quot;size&quot; as the rest of the islands:
(Note - the places I have marked for seams are only general suggestions of where to put them - you will have to find and mark the edges that make the most sense for your object - just try to keep the islands roughly the same size as your other group of islands)