In terms of affiliated supporter clubs, they're actually in the top 5. That said, there are loads of definitions of a big club, and outside of Man Utd and Liverpool there are several clubs all of a broadly similar size and whose fans can realistically dream of semi-regular silverwear.Leeds are a big’ish club. But not near to the levels their fan base think. So many clubs have overtaken them in the last 25 years, by quite a distance. The football landscape has dramatically changed while they’ve been in the Championship & League One.
It should be remembered that Leeds have been in division 3 fairly recently, so they're not that big.