The restaurant should have started using smaller plates to make up for the smaller portions.
Oh, and maybe smaller tables too. 
Here is a joke about smaller portions:
A tourist in Seville is in a restaurant discussing the menu with the waiter, but nothing seems to interest him very much. Then the tourist sees another diner eating a dish of two very large meatballs with obvious relish.
"What is that gentleman eating?" asks the tourist.
"Oh, señor," responds the waiter, "That is the specialty of the house... it is bull testicles, fresh from the corrida."
"Very well," says the tourist, "That's what I would like to eat."
"Oh, lo siento, señor, but you must have reservations a day in advance for such a rare delicacy," says the waiter.
"Ok," says the tourist, "May I come back tomorrow?"
"Oh, sí, señor, come a las cinco de la tarde, and all will be ready for you."
The tourist arrives the next day, and is dumbstruck when he is served two small Swedish-size meatballs.
"What is the meaning of this?!" he shouts. "The other man yesterday had two huge meatballs! "
"Ohhh, lo siento mucho, señor, but sometimes, the bull, he wins."