No impact on the relay. The relay transmits the records, not all of the actual data itself. The video is stored on your PDS. The PDS sends a record to the relay, and that record contains a reference to where the post and video is hosted. The appview receives this record from the relay, and then requests the rest of the data it needs from the PDS.
In practice this is slightly more complicated because for media bsky uses a CDN, which also does store the video
the reason why an atproto relay is so large in storage is because to validate (this is the authorized transfer part of the name AT Protocol) a record it has to know about all previous records by that account, so in effect it had to store the entire network on a relay, which is how you get these massive sizes.
that's what bsky is currently fixing, see this thread for more details https://bsky.app/profile/divy.zone/post/3liwwg45izs2r
there is impact on offering hosting services for a PDS (which no other company is seriously doing right now), that part does get more expensive if you offer larger video storage. Bluesky PBC can eat that cost with VC money, but more difficult as a startup
it also does directly affect costs for running a CDN. So an app like Skylight, which is a bluesky video client that has a tiktok-like interface, if they want to get more independent from bluesky pbc they'll have to use their own CDN at some point, since currently theyre fully dependent on Bluesky's CDN. If they would do that, the upgrade from 1min to 3min video does affect cost
(then again, for Skylight, bsky's focus on video is likely a net benefit anyway, ¯\_(ツ)_/¯ )
