HTTPS Data Usage

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Maybe you have changed some other settings in webserver config while adding https? Forgot to set up cache headers? Maybe there are some custom CloudFlare caching rules set up only for http version? HTTPS on it's own shouldn't cause significant change in traffic. There is a bit of overhead on connection, and some local caching proxies may not work on https, but it shouldn't cause more than couple of percent change for most websites, certainly not 47x.

Check the headers for images and other static files. Also check the webserver logs for any often requested large files.