The zip download function is quite limited in the file sizes that it can download. The process of zipping requires approximately three times the memory of the total file sizes in the zip file (I’m not sure whether that’s total size of the uncompressed files or of the compressed files after zipping, since JPEGs do not compress much further when zipped). The settings for this provide an option to attempt to increase the memory limit when users download zip files, but there’s no guarantee that your host will allow the limit that you’d want to use.
As such, the download function is treated as ‘download of photos’ and not ‘download of photos-for-download’, which can be much larger. I’ll look to add an option to use photos-for-download renditions when available. Once there, I would advise that you test out different memory options in the CR settings against the number of files you allow to be downloaded, to ensure that it works within the memory limits on your server.
This change will most likely be in our next feature release, for which we don’t yet have an expected date.