So today I noticed that piggies only drop one plop, no matter how much you feed them. Unfortunately I found this out when my finger slipped and my pig at 33 of my spinach, but ahem. Off topic.
However, if you feed them things three at a time, you can get multiple plops. So it's not a matter of the game being set so that piggies can only drop once a day.
It would be lovely if the food was cumulative up to three plops for 9 veggies, just like how you can get 3 guano from batterflies if you feed them enough at a time.
After all, where does the other 30 spinach go if your piggy eats 33? o.0