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Registries store layers by content digest, so a shared base image is stored once no matter how many tags reference it. Storage therefore grows with unique layer data per push, not with total image size — which is why 5,400 retained images can occupy far less than their nominal sum, and why changing the base image is the event that suddenly doubles registry usage.
Registry storage
unique data = images per day x new layer MB x retention days; total = unique + shared base; naive = images x (unique + base)
Base image changes, since a new base digest means no sharing with existing tags. Also multi-architecture builds, which multiply layers per tag.
Yes. Untagged images from overwritten tags keep their layers alive. Garbage collection of untagged manifests is usually the largest single reclaim available.