E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Between a pony and a horse in size, from thirteen to nearly fifteen hands high. The word means big, stout. The original meaning is a tuft or head, hence eminent, large, powerful. The cob of the county is the great boss thereof. A rich cob is a plutocrat. Hence also a male, as a cob-swan.
Riding horses run between fifteen and sixteen hands in height, and carriage
horses, between sixteen and seventeen hands.