So, here is a completely unscientific* look at contact patch (float potential!) on a 2.4" 29r tire (red), a 4" (blue) and 5" (green) fatbike tire.
The surface area speaks for itself.
*I roughed out the wheel shape, aligned them all at the same point on the bottom of the tire, then cut through them at the same point... so this doesn't take into account the tire actually deforming better over the surface under rider weight and the reaction from the ground, its just a comparison of the area of that slice... but its telling how many more sq inches you get with the bigger tires. More surface area = better float on soft surfaces.
(click to make readable)
You have nearly double the surface area with the 5" tire, not to mention the ability to run lower pressures. And in reality, the 2.4" tire would not be cut in the same location as I have here, so it would be less (theoretically) - as it is a smaller tire with higher pressure it wouldn't deform as much.