I can understand how a cache works but my other active forums are in the lower MBs. 6.2 GB in a short time is a bit much. I definitely need to keep an eye out for this. I am glad I have unlimited data and 250 GB of local storage.
I just looked at my cache for this forum, its approaching 7 gigs, of which 5.8 gigs was videos. YMMV, my cache for a hobby site I spend time on was only in the megabyte range, but no videos. And you can tell your phone not to cache the site at all if it worries you. But that defeats the purpose of a cache so might actually see an increase in your data usage.