People with type AB+ blood have 3 proteins in every blood cell that O- people don't have, A, B, and Rh factor. Do the former people weigh a few picograms more than the latter people of equal size?

theoretically, I would imagine so, but your weight would be affected much much more by things like how long ago you had eaten how much? was there dirt on you? water droplets?

More proteins, and less of what? There's only so much space in the blood vessels, so if there is more of one thing, there must be less of something else, even if it's just fewer blood cells.

It's not the blood cells, dude, it's the hamburgers! Moderation, my friend!