With regards to
Liam - I never knew until my early twenties when I joined this board that
Liam was short for
William. I never put two and two together, and all of the boys named
Liam I grew up with (it's not too uncommon) were
Liam alone. I don't think it's too obvious to many people - certainly when I've brought it up in real life I've found it to not be widely known.
Different parts of the world do nicknames differently. Australians are, for lack of a better word, simple with nicknames. The name '
Liam' isn't even pronounced the same as the last two syllables of
William, so it seems quite a stretch.