Given the Discly version of a Scottish accent that the Pictsies use, the name Big Yan sounds likely to be a reference to the Scottish comedian Billy Connolly, who is often known as the Big Yin. In Scotland, this is pronounced with the emphasis on Big, and simply means the Big One. I have read somewhere that he was once introduced in England as the Big Yin (emphasis on the Yin) as if a Yin was some strange Scottish creature.
Yan is Gaelic for one - still used for counting sheep in Scotland and Northumbria, the Yan Tan Tethera system. Yin is used in counting sheep in Yorkshire and Cumbria in the Yin Tan Tetra form of counting.
Yan is also possibly a Gaelic version of the name "John". So, although large by Feegle standards, he's still small by human standards, so he'd be a very small Big John. According to legend, one of Robin Hood's most famous outlaws was an extremely tall man called Little John...
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