As an American it makes sense that you hear cannons being referred to by the size of the ball, such as 12 pounders.

Referring to guns by an imperial weight was actually a British tradition, not an American one, or at least this was the case by WW2. Americans in WW2 typically had some intersection of metric and imperial measured guns- a 3 inch gun, a M3 75mm gun, and so on. A brief look at British WW2 era gunnery will tell you that many tanks were featuring 6 pounders and 17 pounders, for example.

Now, I've had this explained to me before but I seem to have forgotten since then. If I'm remembering correctly this was actually a tradition established by the British Navy which was in reference to the weight of the gun, and the weight of the round it could throw, back when black powder was a thing.

Using Pound instead of Kilogram is anachronistic, but accurate, as it was the commonly used unit of weight/mass when cannon were being used.

Metric units were invented in France in the late 18th century, and it would be a bizarre turn of events for the British navy to use a new system invented during a conflict and by the opposing side!