I actually think smaller, old homes, especially in the north, were often built a hell of a lot more solidly than newer ones or larger old ones. There's a very elegant large 1790s house "next door" -- ie, 1/4 mile up the road -- that was built right on the very top of the ridge, and the folks who live there now say it's drafty as hell and the wind in the winter roars right through it up there. My modest little farmhouse half the size was nestled on the SE side of the same ridge, protected from both N and W winds. The farmers who built my place 175 years ago couldn't afford the ego-boo of living in a grand big place on top of the ridge, and they were, I'm sure, a hell of a lot warmer in the winter than the fancy people who built where they could dominate the landscape. (And it may well be that builders in those days were just as willing to take advantage of foolish people with more money than sense as they are today.)
I think you're very foolishly turning an observation of a few individual situations into a very broad generalization. Bottom line, the larger homes were typically built for people who had the option to buy the best materials, and the whim to build it where they chose. The smaller homes were typically built for less affluent, who used what they could afford. Again, a generalization surely wrong in some cases, but also surely more correct than assuming smaller houses and cheaper houses might be for some reason built better.
My house, as well as two very substantially larger pre-Revolutionary houses my family has owned, were built on the southern slope of a hill, nestled down where there is very little wind. When you have 500 acres to play with, you have more options on house location. When I say my house is drafty, it's really a comment on volume of air exchange per size of house, versus a new home with urethane weatherstripping and cellulose insulation out the whazoo. I'm sure it's worse than a modern 6000 sq.ft. McMansion. But, aside from three old doors which have gotten pretty bad, due almost entirely to age, it's actually a pretty tight house for 1773. How many people even have original 240 year old doors on their old houses?
Do note, I named the thread "large or drafty," not "large and drafty."
I don't know anything about old home construction so this may be way off base, but isn't it possible those homes didn't leak as much back then as they do now? It's been over 200 years - I'd expect some "shrinkage" in the wood.
The leaks have just moved, I suspect, but I'd guess most are tighter overall today than they were back then. Structural timber set in a stone wall srinks, but doesn't leak now more than it did then, because the wall has been undoubtedly re-pointed or poridged many times since. What leaks now is old doors that have cupped, or forced out of fitting properly by sloppy repairs and paint build-up, or windows loose and worn. Hopefully storms have been added to those windows, which would have originally had none, and it's exceedingly rare to find an original door of this age on the exterior. Most now have insulated attics, sealed off eaves, and improved other areas originally left loose. Heck, we have a roof you can't see light thru now, which was not a case with the cedar shake roof kept on this house until 20 years ago!