Is this your first android device? If not, I have a hunch that when the phone was activated, Google Restore was left on. Doing so without knowing will cause the play store to download every app that you had on your previous device. So if you didn't immediately go on wifi, it would have done all of that over data. The other possibility is that your device downloaded and installed a rather large firmware update.
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Current Devices: Nexus 5, Asus TF700, Google Glass, (2013) LTE Nexus 7, Logitech Revue, Chromecast
Retired: Samsung Fascinate, Droid Charge, Galaxy Nexus, (2012) WiFi Nexus 7, Nexus 4