Carrying someone bigger than you sounds like a bad idea. However, there are times when it is either necessary or a great deal of fun, depending on your situation. While this person may be larger, the human body has an amazing ability to overcome adversity and carrying a large person is something that can be done.
Bend your knees the entire time. Bending your knees will help you keep your balance as well as preventing you from having a terrible knee injury from locking and bending the wrong way.
Ask the person you are carrying to wrap their arms around your neck. Make sure they have enough control so that you do not get choked. This will help stabilize the person on your back so they are not swaying and slipping, which makes it nearly impossible for you to carry them.
Ask the person on your back to also pick their knees up so that they almost touch your armpits. Here you can put your arms under their legs and lock on like a pair of handles. This will further stabilize the person and hopefully keep them in place while you are walking.
Gather your center of gravity before trying to move. Make sure you can stand still with the person on your back before going anywhere. It is important for the person to remain motionless.
Begin to walk slowly as you normally would. Your steps will be heavier and more difficult than usual, but it will not be impossible. Avoid stairs and hills to avoid injury.