The rules are based on the fact that landing is harder on the airframe than taking off, so as long as max takeoff weight is only a fraction higher than max landing weight you get a pass.
And at any rate you get the greater of 10 minutes or 1% of max takeoff weight per minute to dump enough fuel to land safely. I wonder how long it takes a 737 to circle the runway before it's safe?
20 percent of fuel is used from take off to cruise altitude. On a plane like a 777, that still leaves a very substantial amount of fuel onboard. However on a smaller plane like a 737, you don't have to circle for too long before you're below MLW.