You know, my boyfriend's 3 lbs yorkie does the same, barks like he can actually do something about the neighbour's 60 lbs staff terrier (very friendly dog who wouldn't hurt a fly) in our back yard...
The really funny part is the look on the yorkie's face when the staffie takes him up on the barking and charges him.. Man oh man does he realise quick that he's in over his head and he should find some cover. Did you know dogs could make this face
? lol