Next time you go to him ask him "knsa sasta nasha kartay ho bhai ?"
Now coming to the point the length of the piston has nothing to do with the noise.
Basically a piston has two parameters bore and stroke. The picture below shows what is called a bore and what is called a stroke
The height of the piston is the same distance which the piston travels up in the cylinder.
Now your CD 100 bore is 50mm and Stroke is 49.5mm as can be seen in the specs sheet below
Cd70 piston is out of the question it's stroke of 41.4mm would mean that the piston would never travel the required distance it is supposed to travel and with a bore of 45.6mm would mean that you would have to put it in a sleeve of diameter 4.55mm to adjust the cd70 small piston. Why would you do that ?? I am pretty sure this setup would never work in the first place due to piston height (stroke) being far too less.
Now suppose ustad g was referring to the chonda's piston than it's a possibility. At this point i would like to say that chonda's, cd100 and CG125 piston's all have the same stroke i.e. 49.5mm.
The difference comes in bore which is 47mm (chonda), 50mm(cd100) and 56.5mm(cg125) respectively
Again in order to put 47mm dia (bore) piston of chonda into your 50mm wide cylinder you would have to install a 3mm sleeve.
Why would you want to do that ? Even if you do that basically your cd100's cubic displacement of 97cc would come down to 86cc of chonda's. Why!!!!!! Why!!! plz tell me why ?? (A)
People who own chonda's get their cylinder's bored to install bigger cd100 or suzuki sprinter's piston to increase performance and ustad g is suggesting you to do the opposite and reduce your engine displacement and hence in no uncertain terms reduce power with it.
Are you sure he isn't on some sort of medication ?? :S