Question: Whether 40mm sized table tennis ball spins more or less than 38mm?
About the article "Table tennis", section "Ball".

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  (1981) The Milkor 37/38mm Less Lethal “Stopper” Single-shot  (1981) The Milkor 40mm Single Shot Grenade Launcher  (2008) The 37/38mm or 40mm Stopper Convertible  
The Milkor 37/38mm Less Lethal “Stopper” Single-shot was designed and distributed in 1981 by Milkor (Pty) Ltd as a less-lethal anti-riot weapon along with the Milkor 40mm Single shot Grenade Launcher which was designed as a reduced-cost grenade launcher for the SWAPOL Forces.
In 2008, Milkor (Pty) Ltd released a new design designated the Milkor 37/38mm or 40mm Stopper Convertible, which is a single shot break-open weapon designed to utilize a variety of 37/38mm or 40mm Less Lethal rounds.
As with the original Milkor MGL, the Stopper was originally marketed for Milkor by Armscor (South Africa). 
The international rules specify that the game is played with a light 2.7 gram, 40 mm diameter ball.
The rules say that the ball shall bounce up 24–26 cm when dropped from a height of 30.5 cm onto a standard steel block thereby having a coefficient of restitution of 0.89 to 0.92.
The 40 mm ball was introduced after the 2000 Olympic Games.
However, this created some controversy as the Chinese National Team argued that this was merely to give non-Chinese players a better chance of winning since the new type of balls has a slower speed, while at that time most Chinese players were playing with fast attack and smashes.
A 40 mm table tennis ball is slower and spins less than the original 38 mm (1.5 inch) one.
The ball is made of a high-bouncing air-filled celluloid or similar plastics material, colored white or orange, with a matte finish.
The choice of ball color is made according to the table color and its surroundings.
For example, a white ball is easier to see on a green or blue table than it is on a gray table.
Some Table Tennis balls have ball rated in stars so that more professional players can know which ball to use.
Manufacturers often indicate the quality of the ball with a star-rating system, usually from one to three, three being the highest grade.
As this system is not standard across manufacturers, the only way a ball may be used in official competition is upon ITTF approval.
The ITTF approval can be seen printed on the ball.
The Milkor Stopper is a riot gun used for riot control, designed to fire a 37/38mm or 40mm cartridge, which can be a teargas canister, rubber shot cartridge or explosive.
The weapon is simple to operate and all metal surfaces are treated with a coating for corrosion protection, plus long-life dry film lubrication.
It fires one shot before reloading.
It is a conventional single shot, break-open weapon (opening at the breech like an ordinary shotguns (not pump-action)) with a floating firing pin mechanism that ensures safety when accidentally dropped.
As a security or assault weapon, it can be fired from the shoulder or like a pistol by hand.
The components of the weapon are interchangeable (client dependent), providing a range of applications from Less Lethal 37/38mm to a 40mm Lethal weapon. 
Adding spin onto the ball causes major changes in table tennis gameplay.
Although nearly every stroke or serve creates some kind of spin, understanding the individual types of spin allows players to defend against and use different spins effectively.