repeated variable names are bloat when getting lots of data, extra formatting is the same... CSV's are great for getting these things down.
As another poster pointed out (S)FTP is the way to go when sending CSV's, and they are compressed in most cases to save storage and data transmission on both ends.
I am old enough to remember when mailing a hard disc or a tape might be a faster way to move a LOT Of data. The practice is still alive and well, but the option is now by the "truckload"
Instead the client was given an api to query and no batch processing of information was allowed.
I’ve encountered this idea somewhere, and fortunately have never had to deal with such a broken scenario. Getting piecemeal information is infuriating.
[0] probably an FTP server with no TLS and with the same weak username and password for everyone who connects, a setup which they won't change no matter how many times you ask