The clamping force, and ease of spreading, depends on four variables (assuming the same material): width of the clamp, pipe wall thickness, thickness of material being clamped and pipe diameter.

Width is proportional: a 2 inch wide clamp is twice as strong as a 1 inch wide clamp.

Pipe wall thickness is a cubic function. 4-inch Schedule 80 PVC pipe is almost three times as strong as 4-inch Schedule 40.

Thicker material will have more force exerted on it by the same clamp than thinner material. Think here about how it is easier to spread the clamp a little bit and harder to spread it a lot.

For the same thickness of material being clamped, clamps made of small diameter pipe exert a larger force than clamps made of big diameter pipe.

One way to really increase clamping force is to nest two clamps together.

All that said, with epoxy you don't need to exert much force. Just enough to keep the parts in contact. Even a little clamp will keep flat surfaces together and keep squeezing the epoxy out of the joint until it sets.