You are one of the last people I would ever question about bikes (most of the rest being other people here in C&V) but I have to wonder about that 2.9lbs figure. A radius difference of 4mm means a longer rim and tire circumference of only 12.6mm. Also each spoke will be only 4mm longer, even if there are 72 of them. All other things being equal that wouldn't seem to add up to that much mass. Almost 3lbs??? Nah.
Oh wait, I forgot about the air. Maybe that's it. And your full vs. empty water bottle.
I would suspect that other things weren't equal. Perhaps the nominally equivalent rim was wider or of thicker material for 27". And perhaps the tire was different too.
Sure a weight difference of 2.9lbs would be noticeable. I just don't see how the radius difference alone would account for it all. Now I'm going to have to look up the density of steel and compute it all out. Dang.