I have the exact same issue. I work from home and use wifi via comcast nearly 24 / 7, yet, I get to use almost all of my XL data plan.
This week, I week on vacation and used 27 gigs in 3 days of time, barely using my phone at all.
Back 2+ years ago, I would use the phone like crazy, watching netflix videos and the go and other youtube for hours, and barely reach 1gb of usage.
Now, as soon as I touch the phone to use the road map, It uses 2+ gig of usage.
Give me a break. AT&T, Verizon and other phone companies have found a new way to rip off their customers. My advice: change to another phone network, such as T-mobile or Sprint. By attempting to extort $ 20 - $ 30 extra per month with data usage, they will loose life long customers that spend $ 150+ per months...