A mathematical problem

Persons A, B and C share a kingsized bed.

Person A is 5'6" tall and weighs about 132lbs.
Person B is 16" long and weighs about 11lb.
Person C is 15" long and weighs about 9lbs.
Person A paid for the bed and deals with all the laundry associated with the bed.
Person C randomly attacks persons A and B if they move suddenly in the dark.

Person A seems to end up with less than 20% of the available bed space.

How much space in the bed should person A get and why? Please show workings and any assumptions.