When people say 20 over, them mean the pistons is .020" bigger than stock. .508mm = .020 inch.
These are overbores.
.25mm = .010"
.50mm = .020"
.75mm = .030"
1mm = .040"
ect
If stock piston size is 64mm, and you have 64.5mm, you have a .020" overbore piston in there.