idioms - Meaning of the phrase "Four pounds if he's an ounce"

In The Thirty-Nine Steps, Sir Walter is describing a fish and says "Look at that big fellow. Four pounds if he's an ounce." I've heard similar phrases before but never understood what is being said beyond the emphasis of size.

I've struggled to parse this sentence and I am tempted to believe that it's a contraction of an older phrase. Any clues to the origin as well as the meaning would be much appreciated.

Answer

There's an ellipsis at the beginning of the sentence (John Lawler tells us that this is conversational deletion):

[He is] four pounds if he’s an ounce.

Otherwise this is a perfectly ordinary conditional, described this way in Chapter 10, ‘Rhetorical Conditionals’, of Renaat Declerck and Susan Reed, Conditionals: A Comprehensive Empirical Analysis, 2001, p.345. (Their terminology rests on the stock formula for conditionals, ‘If P, then Q’).

In such examples (in which both clauses typically use an indication measure, amount, number, etc.), the function of the if-clause is to emphasize that there is no doubt whatever that the Q-clause is true. This type has much in common with a direct inferential, since it expresses ‘P is patently true, hence Q must be true too.’ However, the if-clause does not express a real premise, which (among other things) is clear from the fact that it typically follows the Q-clause (whereas premise-expressing P-clauses seldom do). Rather than being the P-clause of an inferential […] the if-clause is a purely rhetorical device to emphasize the truth of the Q-clause.

The fish obviously qualifies as weighing at least an ounce; argal, my assertion that he weighs four pounds is clearly true.