[userpass]

I don't really get it. Sure it's fine if one p2P app uses 3GB (1GB for the append only log, 2GB for a database with indices that can actually be queried) of data. What if you have several apps? Let's say 10. Then you need 30GB and because people only have 32GB to 64GB of storage on their phones the discussion ends right here.

[blurbleblurble]

I didn't downvote you. But your data sizes are arbitrary.

Why would something like a chat or email app need to hang onto that much history?

Imagine a distributed "email" app that uses networks of mutually trusted peers to deliver encrypted messages ("emails") asynchronously. My device doesn't need to hang onto your emails indefinitely. It only needs to hang onto them until they've been received. This could be done via explicitly sending receipts, or probably in most cases by giving stuff simple expiration dates. The sender would have the most incentive to hang onto the original message until its been delivered.

How this scales in terms of MB and GB is hugely dependent on how your application is configured, how frequently new data is emerging, the limits set by peers for how much they're willing to share, etc. But text is pretty cheap. I can't imagine storing 3 GB of yours or someone else's text emails on your phone, short term or long term. The raspberry pi plugged into the wall at your house can has much more storage anyway ;)