Look at the torque converter as a inertial clutch, the faster its made to turn (assuming you have correct hydraulic pressure) the more torque is transmitted to the drive units down the line in the transmission.
Its a little more complicated than that but thats it in a nut shell. the size really has little to do with it, other than cooling capabilities, the turbine, and stator blades can all be changed to perform in almost any way you would like. there is an outfit here in SLC that can buld you whatever you want as far as torque converters go