I found in our R1 houses I could keep about 15 piggies without them getting unhappy. If we expand our yards and our houses, does that number go up? Gladly test myself, but stuck at work, and inquiring minds want to know!
Expanding the yard does increase how many animals you can keep -- the other day I hatched animals (mostly chickens, because of the supplies I had on hand) in my backyard until they started complaining of overcrowding. When I expanded the yard, the complaints stopped.