C2,
As a former manager for a restaurant that will also remain namless because too many people think they SUCK. I've had to deal with substitution problems a few too many times. While I agree in your situation offering an additional piece of shrimp would be in order, it always comes down to food cost. Here are a few of my favorite examples of substitution complaints.
1- Mother's Day 3 years ago, after already serving 2000 people, we ran out of Catfish.

Many of the fried seafood platters available contained 2 pieces of Catfish on them. As a management team we decided to substitute a 9oz fillet of fried Mahi Mahi (Note: I really don't like the way mahi fries up) to replace 2 3-4oz catfish fillets. If i used all of my fingers and toes I still would not be able to count how many people complained that they only received 1 piece of fish. After about an hour of this we had the cooks cut the mahi fillets in two pieces and the complaints stopped. For some people quantity is more important than portion size.
2- Every once in a while after a busy Saturday we would be short on bread to offer gratis on Sunday. (the bread company didn't deliver on Sundays) On more than one occassion I would be beckoned over to a table by a guest. After introducing myself I would often partake in the following discussion.
Guest: We have a problem
Me: What is the problem?
Guest: Our server just told me you ran out of bread
Me: Yes, I'm sorry but we did just run out of bread
guest: Well, I want some kind of compensation!!! That table over there has bread
(At this point I try not to laugh at the notion that this person is asking for compensation for something for which they were never going to pay for in the first place)
Me: They ordered a loaf of garlic bread, sir. Would you like to place an order for one?
Guest: How can you have garlic bread, but no plain bread?
Me: We prep the garlic bread in the morning before we open.
Guest: Well then yes I want a loaf of garlic bread......but I want you to pay for it!!
(As you can guess this conversation could go on forever. At this point, usually, I would comp and order of GB. In some cases the guests would finish that loaf and then have the cojones to request another. Just remembering this raises my blood pressure)
Flip
"Beer is proof God loves us, and wants us to be Happy"
-Ben Franklin-