The large doughnut can mean mean many things depending on the context.
However in this context it merely means an oversized doughnut, with the added capabilities of flattening things, e.g. a chav. And that's always a good thing.
Person 1: "Hey there's a large doughnut rolling down that hill."
Person 2: "It flattened that chav!"
Both: "Yay!"
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