As someone who doesn't develop video codecs, the bandwidth consumed by sending multiple video resolutions seems a poor trade-off for the bandwidth saved by receiving a reduced resolution. (This isn't a criticism, but curiosity - I'm certain the developers thought of it.)

Is it possible (or is it already done) to compress the data by sending the lowest resolution in full, and then for each higher resolution, send only the delta between it and the one immediately beneath it (i.e., resolution layer 5 would contain only the delta between it and resolution layer 4)?

Perhaps because of E2EE the final output would have to be reconstituted on the fly client-side (i.e., because the server lacks visibility into the data), which might be asking too much. Then again, the client is generating all these resolutions.