Lightroom 6.1 would not zip catalog during backup on Mac if it is more than 4293656963 bytes (4GB).
This is because the default Mac OS unzip utility cannot unzip such compressed catalog. It would complain that such zipped catalog as "corrupted".
However, some 3rd party utilities, such as Stuffit Expander, can unzip such kind of zipped catalog just fine. But Lightroom cannot assume that the users will have access to these 3rd party programs. Thus we decided on such Lightroom backup behavior for catalogs that are bigger than 4GB.