"Okay, so he gets to make a dexterity saving throw... That's a 16, so he manages to sidestep you and you take the full damage. ...No, he doesn't have disadvantage, you made a huge shadow above him, he knew you were coming. Anyway, he continues his monologue, using your actions as an example of the foolishness of humanity."
Call it what you want. By RAW, it's a DC 15 Dexterity saving throw to avoid damage regardless of the size of the falling creature (beyond that if it's tiny it does no damage at all). And that's assuming you're playing with the optional rules for creatures falling on other creatures causing damage.
This isn't a physics sim. This is a roleplaying game. Physics are what I allow them to be and it's funnier for falling creatures to have a drop shadow.
If it's large enough to impose disadvantage on a Dex save it's large enough to cast a noticable enough shadow to grant advantage, therefore a flat roll.