Consumer surplus before tariff: red plus green plus purple plus light blue plus dark blue

Consumer surplus after tariff: red

(Domestic) producer surplus before tariff: yellow

(Domestic) Producer surplus after tariff: yellow plus green

Government gain from tariff: light blue

Why there is a net national loss: the nation consists of consumers plus government plus domestic producers.  So with the tariff:

So the green area just passes from consumers to domestic producers, but is not lost by the nation.  The light blue area  passes from consumers to government, but is not lost by the nation.  However the purple and dark blue areas are lost by consumers and not gained by anyone else, and there are no other gains anywhere else.  Hence there is a net national loss, as there will always be in the small-country case.