Improve Level Streaming Size Map - Disk Or Memory

When optimizing for games that make use of multiple streaming levels, should you be looking to reduce size on disk, or memory size? We had a lot of high res textures made for our project, and have been using the MaxResolution setting in the texture editor to lower the resolution to something more manageable. This helps us keep the loaded memory size low, but the on disk size is still large.

My assumption is on disk size wouldn’t matter too much once it’s on disk, and it wouldn’t impact load times. But I’m starting to think this is wrong, and the size on disk is the most important factor when optimizing for level streaming. Is this correct?