You're assuming that all other constants for the hour glass comparisons are constant. In theory, an hour glass could be rendered usable if more sand was added to the device on the condition that the bulbs are increased in size. I may be wrong (because I don't know how the physics work with the added pressure on the sand near the bottom of the bulbs due to the added weight), but wouldn't the neck width not need to change as much (porky doesn't want to make the life end faster, or else upgrading hourglasses is useless!), thus rendering the metaphor to hint at a body build like Jessica Rabbit?