You know how in paintball limits are usually like 250-300 FPS.... Well it's mainly because of the energy that the paintball has. That is why we can play with 400 FPS outdoors (using .20 gram BB's) because it's a lighter projectile being shot and to achieve the same energy it needs more speed. Now that's out of the way, if you do the math and note that a paintball has significantly more energy then you're right. BUT there is the point of the paintball actually breaking and losing a lot of the energy on impact (momentum loss through destruction), whereas an airsoft BB is a rigid object.
SO what does a heavier projectile do for you if all it does is slow down the speed? Well.... It will do 3 things. First increase your EFFECTIVE RANGE, second decrease your TOTAL/OVERALL range, and third make the BB more "accurate/precise" due to a heavier object needing more force acting on it in order to deflect it to a different flight vector. Now why does it increase effective range but decrease overall range? Think about it this way.
If you had 3 baseballs of different weights, lets say 3 oz., 5 oz., and 7 oz. You throw each of them in succession with the same energy/effort at a plywood target on the other end of the field or something. You're aiming true and not correcting for anything and there's a light squall/gust crosswind. You'll find the 3 oz one although you can throw it faster is more affected by the crosswind than the others. The 5 oz one is slower but because it has more mass is less affected by the crosswind and 7 oz least affected. Now about ranges.... You agree that the lighter an object is the more velocity it has and therefore can travel further before gravity (a "9.81 m/s" pull downwards to the ground) will bring it down to the ground. So ignoring accuracy/precision you try to throw the objects as far as you possibly can, you'll find that the lighter one will go furthest but also by the end of the flightpath go in crazy directions because of the crosswind, same with the heavier ones the 5 oz one will go further than the 7 oz totally but then it will go in a crazy direction.
I know it's a big wall of text but I hope this analogy helps.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE