Free hosting on relays is the problem, it's faster and more reliable to store on the device than fetch. I guess there should be an alternative so it's not using 6GB locally though, which is excessive.

I think IPFS is a possible good match for Nostr because there's currently tons of free hosting thanks to the excessive money flying around the web3 world (see nft.storage for free and long term guaranteed IPFS / Filecoin hosting)