It is the iphone 5
There are posts on this forum where there is excessive data use due to cellular data being used while on wifi, or via apps constantly updating etc. You have to turn cellular data off while on wifi and some people said to turn off apps from updating over cellular data, there is also mention of excessive use while having icloud services running.
However Verizon is aware of the issue but why would they help you? They make a huge profit in your data usage going over your allotment or on having you change your data usage.
Yes its a problem with the iphone 5 models.
My wifes 4s has never had this problem... in over 2 years of use. She HORDES DOZENS of free apps, never even closes them out from her screen top. She plays free-app games all the time when on my wifi network. Yet her data usage is a fraction (1/6th - 1/8th) of what mine would be on my 5c... EVEN IF I NEVER ACTUALLY OPEN / USE AN APP.
My YT video commentary here: