318 .40 Over..?

I'm not much of a gear head, horsepower guy.. I just don't have the knowledge.. I bought a 71 318 a couple years ago.
I have not heard it run, and it's been on a stand since purchase.
I was "told" it ran good / strong.. No details given on the rebuild, but it does have a double roller chain and j heads. I'm putting a LD4B intake on it. No idea on cam specs.
I recently took a look inside the cylinders and found the .40 stamped pistons.
I guess my main question is, what is the reason someone chooses .40 over .30 or .20..? Why not .60 over..? May be a dumb, rookie type question, but just wondering.

Thanks.