Q: How can I tell why my iPhone 6 is using so much data

What do you mean by data? Do you mean using your data allowance from your wireless provider, or do you mean storage space for files?

See how much storage you've used on your iPhone, iPad, and iPod touch - Apple Support - http://support.apple.com/HT201656

Hi  Timtash about to cellular data case.  About cellular data settings and usage on your iPhone and iPad - Apple Support