My guess is that either the recipe used in Canada is different, or Canadian law requires them to give data for the *average* serving size. In the US, they're required to list the nutrition for the legal serving size-- the smallest amount they advertise.
It's illegal to short people on what you serve them. If you advertise that your ice cream cone is 5 ounces and somebody only gets 4 1/2 ounces, you can have legal issues. To avoid that, fast food places make the containers way oversized, so big that their stingiest or stupidest employee will still put in at least the legal minimum. It's cheaper to give out extra food than to train your employees and/or measure what you serve. If you advertise 5 ounces and everybody gets 8 ounces, there's no worry about the legality. That's in the US; Canada may have an upper limit as well as a lower one.