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I measured the down tube on both bikes, and they are in fact discrepant; however, I spoke with the manufacturer, and the rep assured me that down tube length is nothing to investigate as a possible source of my discomfort while standing on the smaller bike. It seems odd though that the two bikes (the way I have them configured) are nearly identical, except for down tube length. The bike with shorter DT length = terrible weight distribution in standing position. The bike with longer down tube length = engaged core/weight on legs in standing position.

NIce article. I've read every comment here, and I have a riddle that may relate to General's and Big Bird's assertions re: downtube length. Here goes: I've got BOTH a Superfly 17.5 AND a Superfly 18.5. I've set them up so that the reach is exactly the same on both bikes. The 17.5 (with stock stem) wrecks my back and neck, and I believe this back and neck discomfort results, at least in part, from the standing position; when I stand on this bike, I can't seem to get my body situated so that my legs support my weight. Rather...