Hemingway's central character, Santiago, is using a colloquialism common to hunters and fisherman, especially those seekin larger game animals or, in this case fish.
A fish (or game animal) freshly caught will weigh considerably more than when it is prepared for eating. A "dressed" fish is one that has at least had the entrails removed, possibly had the skin and head removed and will lose significant weight. Santiago is simply saying that a fish that weighs 1,000 pounds after being so "dressed" would have probably been several 10's if not more pounds in weight and would have been an impressive fish indeed.
The answer to the last part of your question lies in the quality of informal speech used by the native Santiago. Additionally, "dressed out" is actually a compound verb and as such, takes no direct object...
One of my favorite Hemingway works... often seen as somewhat autobiographical by this unique author...