" In surfing the wave and the rider are the motor. Mass is just mass. And more mass means it takes more work from the wave and the rider to move. 
" Greater mass ( all else being equal ) gives greater acceleration and a higher top speed as well as more momentum.
Clue: When the wave lifts board and rider it imparts more energy to those with greater mass. There's much more power in even a very small wave than is ever used by the surfer, that's why small waves can lift supertankers.
The power exerted by the wave lifting the rider is in the range of 3 to 10 horsepower depending upon the mass of board and rider and the speed with which the wave lifts.
In contrast the power exerted by the rider is only about a fifth to a third of a horsepower.
Adding 20 to 30 pounds of weight to the board and rider will increase the power of board and rider when lifted by at least as much as muscular energy available from the rider. It's not quite as simple as that because an increase in mass also gives an increase in inertia, but there's an improvement in the thrust to drag ratio with the increase in mass.
Of course the foam board industry don't want you to know this, and it's why so many foam board riders find themselves confused when roasted by riders on big heavy boards.
Big heavy boards also lower the centre of effort and the centre of gravity, which is why we rider of big heavy wooden boards can drink cups of tea in situations where foam board riders are playing 'parachute', 'pearl diver', 'TOAD', 'gosh I'm being left behind', 'surf's sh*t so I'm hanging in the carpark', 'we don't care that we only went 10 feet while you went 500 yards', 'stinkeye', 'weep into my beer at the sad boys club' or other foam board rider games.
