The texture allocator assigns each layer a slab size. If only a handful of items of a certain slab size are allocated, the full 512x512 region can't be used for anything else.

We would benefit from being able to have multiple smaller regions per layer, for example 4x 256x256 regions on some of the 512x512 layers while retaining 512x512 for the items that are larger than 256 pixels, to reduce the average amount of unused space.

Bug 1679751 is a better solution to the problem.