20" Bass Drum and 15" Tom Tom

I would love to know the reasoning on these two odd sizes of drums that Phil chooses to play on his kit. A 22-24" kick is about the norm and a 15" tom? I really don't know of anyone else playing one of those other than perhaps Nicko McBrain.

The 20" bass drum he always put down to feeling more natural due to his height.

As for the 15", I would suggest to create a deeper sound, so rather than 8, 10, 12, 14, 16, 18, he went with the 8, 10, 12, 15, 16, 18?

It is also quite unusual to have an 18 final tom, but also lends itself to the deep sound.