Zip won't help much when compressing lists of pubkeys.

Even for a mobile client, uploading 67kB is not much, especially compared to what relays send in reply. If you want to make your client more responsive on startup, maybe you can query frequent posters first, to get something on the screen and send the full filter later.

If you consider involving the relay ... maybe we can reference collections of pubkeys via list events? The relay would get the relevant list event via it's naddr and then expand that list into the original query. Instead of {"authors":["c88821bdafd6c608107c9c4969672b57c3a8c85f2a0f6bc7b749c3c5609b3424"]}, {"authorLists":["naddr1qvzqqqr4xypzq3huhccxt6h34eupz3jeynjgjgek8lel2f4adaea0svyk94a3njdqy88wumn8ghj7mn0wvhxcmmv9uq3uamnwvaz7tmwdaehgu3dwp6kytnhv4kxcmmjv3jhytnwv46z7qqlgd6hyun9de6zq3n0d3kx7aeqf35hxapvyqcnzte3xqhnyvpjxvx3tr5w",...]}