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.

Edit:
Wikipedia (that font of all perfectly accurate information at all times ) gives a range of density for steel as between 7.75 and 8.05 g/cm3. So assume a high value of 8, or .008g/mm3. A 2.2/2.0/2.2 spoke has a radius of only 1mm, so the cross-sectional area of the inner length of spoke (the part likely to be longer for the bigger wheel) is PI * 1 * 1 mm2, so the volume of 4mm of spoke length is PI * 4 mm3. Multiply this by .008 then by 72 spokes and you get a total mass differnece in the spokes of only 7.87g. That's less than .02lbs. Surely the extra mass from the longer circumference of the tires and aluminum rims doesn't weight much more.

If I was at home now I'd just go weigh one of my wheels with tire mounted. Betcha' a whole front wheel doesn't weigh enough to account for a pound loss, unless the smaller rim is made of an alloy of lighterthanairium and negatimassium. (They are kinda' scarce around here.)