Small Plate most commonly refers to a plate filled with a small amount of food, which the person intends to refill several times before the end of the meal.
At a restaurant a Small Plate is a nicely presented dish of culinary delights which is so tiny that it's sure to leave you starving for more. It also costs exponentially more than the actual value of the food.
Small Plate may also mean a snack chosen out of lack for better options and a larger plate with more delicious food should follow shortly.
"We have a special today, its a small plate, and comes with a bite of local cheese and one organically grown nut, for only $8.95"
"I guess ill have this small plate of leftovers while the feast is being prepared"