Pay by the Cubic Inch/Weight Ratio Air

So on a plane is weight or space a bigger deal?  Weight consumes fuel while the space you and your luggage take up limit the number of people that can fly and thus money from ticket sales.  Well what if you had an airline that actually charged you for how much mass you had?  All the skinny or small people would fly that airline and love it because they plenty of space around them (due to only skinny small people sitting next to them).  The seats could be closer together and you could get more on each flight.