Ben:

This is as fundamental as it gets. This weight variance is normal among double rifles, and you're trying to read way too much into it.

It's sometimes said that the lower quality guns are the most likely to be overweight, but that isn't always true. Some time back I ran across a superb Westley Richards best quality sidelock ejector in .450/.400 3" NE. This was a fairly late rifle from the '30s. It was 12 1/4 lbs. Just last week a friend called about an early Jeffery in the same caliber. It was 12 lbs, 2 oz. My own rifle in that caliber is 10 lbs, 2 oz. A friend just bought a really nice Holland .500 BPE hammergun that's around 9.5 lbs. We previously looked at a mint Purdey .577 BPE hammergun - that was 9.5 lbs.

You will see the same thing in all calibers, black or nitro. NEVER assume that an unusually heavy BPE was intended for nitro. That's what proof marks are for. I'm always shopping for several folks and probably turn down more otherwise nice rifles for excessive weight than for any other reason.

Especially among the British gunmakers, the idea was to build the gun heavy enough for the given caliber to make it tolerable to shoot, but not everyone agreed as to what the ideal was. Keep in mind that the customer ordering the gun may have had his own idea. And yes, some were built on actions of excessive size for caliber in order to save money.