The memory contents have to go somewhere. They can't stay in RAM as RAM will be powered off. If there's sufficient free swap space they go there. If there isn't, the current swap and memory contents get compressed and it then goes to swap. If there's still not enough space, hibernation will fail. So if your swap is filled with incompressible data to its full capacity, you will not be able to hibernate. This is very unlikely if your swap is enormous, but it can happen.