Dictionaries are descriptive, not prescriptive. They try to write a coherent, understandable description of what made the British Empire and the Romans empires while clearly leaving present-day Italy and Britain not-empires.
Current-day Italy can boss San Marino and the Vatican State around, current-day Britain has a scattering of islands around the world. Current-day Italy indubitably has the military force to beat San Marino, Britain did actually win back the Falklands but its victory didn't seem self-evident at the time.
When I squint at the map, the US foreign dominions and military bases look a little on the large side for a plain country, but ridiculously small for an empire. The US military is all over the world and has very impressive weaponry, but was beaten quite thoroughly by the Viet Cong, hasn't beaten the Taliban and and gave up after the first attack by Hizbollah.
"Empire" in quotes looks quite appropriate to me.