Ok, it is an interesting point, and perhaps even related to the topic of the thread: do passive neutrally-buoyant drifting objects move at the same speed? Yes, is the answer, but those objects are rarely passive....and I guess your point is that a little movement from a big person has more effect than a little movement from a small person.That bigger people have an advantage moving themselves through a draggy medium (water, air, whatever). This is somewhat to the contrary of what the OP believes they have observed and is to the contrary of SeaRat's statement that size doesn't make a difference under water (which I don't think he really meant exactly like that).