iOS 9.1 beta 3-4 eating up my wifi downstream
I would do three things to narrow down the culprit:


Connect to Xcode and look at the console logs for the phone - activity might be logged
Connect to Instruments and profile the network activity
Connect to iTunes and make snapshots of the storage sizes (or use storage and usage on iOS) to see what bins are growing over time.


You can turn off iCloud to reduce many downloads, but it really depends on which apps you have installed, how they background data load and what data stores they are configured to pull. Since you are on the betas, you have many options to pick apart the data flow using diagnostic profiles provided by Apple and the developer tools similarly provided.