"Sweating like a pig" is a derogatory comment, meaning one is sweating heavily and disgustingly. Pigs do not sweat, and they have no body odor. They have a few sweat glands on the edge of their snouts. So, I guess if your nose is sweating delicate little droplets, 你 are indeed sweating like a pig.
I think the reason that people say "I'm sweating like a pig" is because 你 would think pigs would sweat, being a large animal and the fact that people say they smell. However not many know that they don't - sweat I mean.