My understanding is that the large, full, complex Paphs are called "Bulldog" Paphs because of their parentage. Back in 1951, the Stuart Low Company created Paph Winston Churchill, one of the most important complex Paphs ever created.
Later, Winston Churchill was crossed to create Winston Churchill 'Indomitable' and Winston Churchill 'Redoubtable', also reddish complex crosses with spotted dorsals that are still important in breeding today.
In 1979, Paph Small World was crossed with Winston Churchill 'Indomitable' to create Paph British Bulldog, also a reddish complex cross with the same flower shape and fullness of the Churchills.
All of these Paphs had similar characteristics of fullness, colouring, and spotting on the dorsal. My understanding is that collectively, all reddish spotted, complex hybrid Paphs came to be referred to as Bulldog Paphs because of their parentage and that the term later was casually expanded to include any complex Paph that has large, rounded flowers similar in shape or appearance to British Bulldog, or any of the original Winston Churchill plants.
As it has sort of become Paph mythology, other versions of how the Bulldog term has come about may also be circulating.