I would return them and look for another pair that fits, local to you. If you can't do that then sizing charts and reviews will be your best friend. Sometimes reviews will tell you how true to size the pants are which is always helpful.
Have you tried the pants on with the rest of your gear, strapping in, maybe fool around in the back yard/living room to see how it feels (assuming this is an option)? Personally, I wouldn't be comfortable in pants that big but some people prefer it that way.