Heavy objects, on average, are denser at human scales. Dense objects tend to have an aerodynamic profile compared to sheets of leaves or cloth or sand. They tend to get blown away in the wind. Anything that would bind or compact those lightweight things together like resin or water tends to weigh a sizable percentage of the compound. There is a correlation between heavy and fall speed. It took accurate scales and ~0 bar vacuums to prove it was the air doing it.
Try explaining how a helium balloon works without sounding like a wizard.
They tend to get blown away in the wind
True, but not because of “aerodynamic profiles”. Create a sphere out of lead and one out of styrofoam and the lead one will land first. The real difference is air resistance. Probably the first piece of physics anyone learns is f=ma, and this tells us that with the same force (e.g. the same amount of air in the way when travelling at the same speed), a lighter mass will experience more acceleration (in the case of air resistance, less acceleration in the direction of fall, because of more acceleration in the upwards direction) than a heavier one.
There’s something to the aerodynamic profiles though. Less surface area results in less force being applied. So a flat sheet of paper falls slower than that same sheet crumpled up. The things mentioned are light and have lots of surface area for the wind to apply force to.