Your example goes against physics, really don't get into this arguement you will be proven wrong.
Your not only working with races that are built far differently from human beings, even within humans your size/build can be massive factors in how high you can jump, how fast you can run, swim, almost anything. Bigger doesn't always mean better but having a hight advantage with a small frame will put you at a advantage as someone shorter with the same type of frame.
Within XIV we have a large ammount of races that are generally the same size. Hume, Highlander, Roe are generally the same size with differing frames. To say that a perfectly fit Hume can't jump higher than a perfectly fit Roe is just asanine and you know it...but let's not even go with the comparable races.
Take a long hard look at the Lalafells legs compared to ANY other race in FFXIV, they litteraly lack the proper proportions to jump high at all they would get next to no leverage or momentum with the extremly small legg, they lack the muscle mass as well to propell themselves. The only advantage they have is Weight but they still have inferior mechanism to launch themselves with.
Physically Lalafell are "Top heavy" they have much larger torsos than they do legs, If they were equally sized even in leg/torso/build you still have the taller race that has a advantage in height advantage giving them more "Clearance" even with the same physical abillities.
It's "Video game logic" that Lalafells can wield the same axe that a Elzaan can and output the same ammount of force (Damage) So is the abillity of a shorter race being able to jump as high as the taller ones. It's a balance issue if they did it realistically. I'm done arguing this stupid point.
Don't beleive me if you don't want too, but it doesn't make you any less wrong than you currently are.
Feel free to make the same point about races that don't have such a massive difference in build. It was a "Lolnotserious" example much like your Aero spell...Edit: Actually, now that I think of it, your point can go against what you're trying to make. Theoretically, the body builder with his height and mass could not get very far off the ground in relation to his body. The child does not have as much mass holding them down, but they are much smaller. The distance between the ground and their feet will still relatively be the same.