A lot of the reason wood is heavy comes from moisture content. An easy way to experience that is to visit the pressure treated lumber section of a home improvement store. Some of the boards will be very dry and light. Others will be obviously wet and much heavier.

Wood needs to have most of the water removed(drying on a huge oven called a kiln) or by leaving in the air and dried naturally. Kiln drying takes a day or two, air drying can take a year or more.

I have moved a lot of bundles of wood flooring, but never found any bundles really much heavier than the others, since the wood should have been kiln dried.

However, it is possible some of the bundles have been exposed to water(rain/snow) if improperly stored. A tool called a moisture meter measures the moisture content of a board. Boards that do not have basically the same moisture content as the rest of the stock should not be used until they dry to the correct level.

Thanks. I think I am getting too tired hence the crazy weight (well, feeling of).

I am worried about the boards I have to leave in the garage overnight, getting more moisture in them. I physically can't do anymore though - for today.

This link gives you the info. Moisture does not necessarily change levels in a day or two. Cold temps are better(less moisture absorption) than hot, humid temps. But, the flooring should be stored in the house in the conditions (heated/cooled) that will be present when living in the home.

Okay, now reading this article, when it comes time to install, should I be worried about the new subfloor that I put in? Should I also let that sit for a few days to let the moisture out of it prior to putting the floors on top?

You should buy or rent a moisture meter to insure that your subfloor meets the dryness specifications that your flooring manufacturer set. (it should be on the box) If your subfloor is very wet for some reason it could take a month to dry to the right level -- that would be pretty unusual. It's always a good idea to let the flooring equilibrate for a week or so in the conditioned environment where it's going to be installed. I cut the bundles open and lay them (the bundles) out in a single layer to that air can circulate around them.

Variations in weight also happen if the boxes have different amounts of heartwood versus sapwood in the boards.

If you are really curious, stand a few boxes up on a bathroom scale and weigh them.