jq is able to easily process jsonl as well as regular json.
All you need to do is add -s option to read whole input and merge it all into
a large array.
For example:
$ curl -s https://kojipkgs.fedoraproject.org/repos/eln-build/6071552/x86_64/rpmlist.jsonl | jq -s '[.[] | .size] | add'
105368819406