>Grenades were never 3-4lbs, what you are talking about is mortar/cannon shells.

I'm pretty sure a mortar or howitzer shell during the Napoleonic Wars weighed much heavier than 3-4 pounds. A shell for a 6 inch howitzer of the French Gribeauval system would had fired a shell that weighed at least 20 pounds. For a large 12 inch siege mortar, it would had been around 150 pounds.