Why does the washing machine leak only during the tumble dry cycle

Because you mentioned the load size switch, it sounds as if it has issues detecting the empty tub when set to a minimum change level (small load); this may indicate a blockage of some sort.

There is a thin 1/4 inch tube that runs from near the bottom of the tub up to the water level switch inside the console. Disconnect and blow the gunk out of that tube. You will need to get inside the console and the front of the machine must be removed to do this. Use a putty knife to the front, then unscrew the console.

If you had mentioned the speed switch making a difference, it would be the motor/transmission or something stuck slowing down the assembly.

You have a crack somewhere in the drainage system. The most likely reason that it's leaking on the spin cycle is the vibration caused by the spin cycle is opening the crack to let the leaking occur.