While some dog breeds are very large, like the English Mastiff or the Great Dane, they are still significantly smaller than even the smallest horse breeds.
The tallest dog breed, the Great Dane, can reach up to 32 inches tall at the shoulder, while the smallest horse breed, the Falabella, can be as small as 17 inches tall.
So, while there are some very large dogs, they are still much smaller than even the smallest horses.