Most current "wall wart" adapters are similar, they contain a switching type power supply as opposed to the older linear type. The switching supplies are much lighter in weight, mostly because they don't have the heavy AC transformer that the linear supplies do. There is a transformer but it's much smaller/lighter, and the rest is all electronic components that don't weigh all that much.
The OEM adapter for my Asus Eee PC is almost identical in size, shape and weight to the one that comes with the Mini.
I doubt anyone is going to want to open one up unless they have a spare lying around as a throwaway, since they're sealed and any attempt to do so will be destructive.