Decompressing an asset so that it can be used is an operation that takes processor cycles as well. It's why Titanfall 1 came in so high on storage requirements at the time, because in order to meet CPU performance targets, they had to leave audio uncompressed. In this case, huge texture asset files are often LODs, high detail versions for when you're up close and low detail versions for when you're far away, so that the machine is always loading the right size version of the asset rather than just always using the best quality one in a worst case like you seem to be implying. This takes up a lot of storage space, but it means they aren't wastefully using high detail assets for a mountain a mile and a half away.