What is it?
They are testing big blocks in another chain or simulator, or what?
this is a testnet for segwit. before a release to public or on the mainnet Core is testing all the improvements.
Do you know how much of it is witness data and how relevant this would be to the kind of usage patterns we have today? Are these kinds of segwit block sizes perhaps more relevant for when LN is launched?
The kind of usage we have today doesn't fill a 1MB block segwit or not.