I don't think this correct - in fact, I know this isnt correct!
When in "online" mode, but not directly connected to another user (i.e. on a call), Skype doesn't use much bandwidth at all. It does use some as it is constantly checking your contacts to see status changes etc, but that is minimal. One figure I've seen quoted a few times is approximately 300Mb per month.
Again, just from figures quoted by online users, video calls use 200-300Mb of data per hour and around 50Mb of data for voice calls.
This would suggest that if its a standard install, Skype itself is not your issue. Someone streamed a couple of episodes of Corrie and not mentioned it per chance?
JF