Question PSD Importer generates too large texture

Hey, we are trying the 2D PSD Importer from Package Manager.

It does a good job except that it sometimes will generate a much larger sprite sheet texture than actually needed, say, half of the texture is nearly blank. We think this may due to Unity tries to make the texture be POT so it's compatible with the default compression format. However, this will dramatically increase the download size if we build it as asset bundles.

Besides, we are considering to change the compression format to ASTC, to make it able to keep an arbitrary dimension and a relatively small size. By the way, are we able to write some preprocess code to overwrite the default import settings for psb files? Like change the texture format?

Is there any way that we can control the generation process or rearrange the sprite rect of the sprite sheet texture?

Thanks.