The reason the Europlug is rated 2.5A is quite simple, it is designed to make contact with quite a few designs of socket outlet, most of which are in fact designed to mate with much large 4.8mm diameter 16 amp plugs.
Europlug's pins are bent slightly inwards and somewhat springy to ensure that it makes good contact, but contact is only made on quite a small surface area of the pin, on one side only. Where as the 16A counterpart makes contact on a much wider surface area of the pin.
The Italian 2 pin plug on the other hands is designed to make proper contact with Italian sockets, so it can be safely rated 10A as it's a complete system.
