Fact: oceans used to be a lot colder, back in the ice ages. Fact: shrimp shells are made of shrinkable material. This material is solidly attached to the shrimp, though you might not think so the way the shell comes off so easily when you cook a shrimp.
After the last ice age, the oceans warmed up. The shrimp outer coverings shrank, like cotton shrinks in hot water, compressing the shrimps’ insides along with them. That’s why shrimp are small.
But there’s more. When the shrimp insides compressed, all of the flavor got packed into a smaller space. So you have more flavor per square centimeter. That answers an additional question: why do shrimp taste so good?