To be more precise: fuel efficiency standards go down with the physical volume a vehicle takes up.
So every year efficiency requirement goes up, but you just update the body every few years to add a little more sheet metal and stay within your legal mandate.
Larger vehicles don't have lower safety requirements, that's just patently false. They're doing it for emissions compliance reasons as Koala said.
They have the same requirements, and need more much reinforcement to make up for all that added mass. Most of the NHTSA's tests involve either a vehicle of a set size running into the test vehicle, or the vehicle under testing to run into a wall. A heavier vehicle is going to need a lot more reinforcement to reach the same level of protection running into a wall than a lighter one.
They are less safe for pedestrians, but those requirements are all more or less the same regardless of size. Manufacturers aren't deliberately trying to make it less safe for pedestrians. They just don't really put any effort into it other than meeting those requirements, and making the "best" car outside of that.