If Safari isn’t downloading larger files, that’s not zip or unzip. There’s something else going on.
Any add-on anti-malware, add-on anti-virus, add-on cleaners, add-on network firewalls, or add-on VPN clients here?
Does this same issue arise when booted in Safe Mode? That rebuilds caches, and disables various extensions and add-ins.
Download and run and post the output of EtreCheck here. Open a new reply here and then press the button that looks like a printed page to get a text input box big enough to paste the hardware and software configuration report here.
Once you have a complete zip archive downloaded....
Make absolutely sure whatever is zipping this is minimally running zip 3.0, and minimally unzip 6.0.
zip -V
unzip -V
Earlier versions of zip and unzip don’t play well with larger files.
But with a failing Safari download, you haven’t gotten to unzip.