On-demand programs have often been run through multi-pass compression cycles to produce more compact files.
Watching live events as they occur requires getting the stream processed and out the door as quickly as possible with no time for multi-pass processing, so they tend to contain more data to stream.
Whether ESPN then optimizes compression on its recorded live events for replay, I don't know.
Thanks. I was watching Thursday Night Football on Prime and it was perfect. At least 720 and mostly 1080.
I've been dealing with DSL (2.5Mb) and cell modem (10Mb) for years. I'm on the wait list for Starlink which I assume will be much better.