Cows are large animals, typically weighing between 1,000 and 1,500 pounds. Dogs, on the other hand, can range in size from a few pounds to over 200 pounds, depending on the breed.
Even the smallest cow breeds, like the Jersey cow, are significantly larger than the largest dog breeds.
It's possible you might be thinking of miniature cows, sometimes called "miniature Herefords" or "Dexter cows," which are smaller than standard cows. However, even these miniature breeds are still much larger than any dog breed.