Bigger Bells?

While i was doing my workout yesterday i was thinking that i might get two bigger Kettle bells. At the minute i use two 16 kg bells. now they say that they weight that i should be using is 20 kg! i was in a sports shop and i lifted a 20 kg Beast, Jesus it was heavy! but then i remember it being the same when i got the 12 kg , and now im using the 16 kg every day. what do people think? in order to increase size and strength do i need to up my weight? should i be using 20 kg anyway? If so does anyone know where i can get two?