The Big End is the end of the Connecting Rod that links to the Crankshaft
the Little or Small End is the joint between the Piston and the other end of the Connecting Rod.
Big End failure (also called 'running its bearings') is usually due to running an engine with low or without oil at all... May also happen if the wrong grade of oil is used...but dont hold me to that.
@NBC - Yes, thats a new Engine mate...You can use the bottom end from a Cento Sporting if its a '55 Punto.  But its easier just swapping the whole engine!!