@nemobis atproto relays currently mirror only "records", not media blobs, so size isn't too crazy.
we think a degree of duplication and mirroring is good/healthy for the network. similar to having multiple copies of git repo checkouts. but a few dozen full network copies is probably plenty?
resource/climate-wise, what we see in our infra is that "reverse chron" timelines, and to some degree notification tracking, are expensive (much more than relay). ironically "algo feeds" are cheaper?