How do smaller gaps between tread cause a regular tire in the mud to resemble a slick tyre?
Paddle tire:  Smaller gaps trap the mud in between the tire lugs, which turns the tire into something resembling a "slick" with no tread at all, which will have no traction in mud. Racing slick: A racing slick (also known as a "slick tyre") is a type of tyre that has a smooth tread used mostly in auto racing.
Smaller gaps trap the mud in between the tire lugs