Falling objects would deal damage determined by size, not falling distance.
Winged kobolds actually make use of dropped objects as a weapon. Note in the description how damage doesn't change based on how high the rock is:
Dropped Rock. Ranged Weapon Attack: +5 to hit,
one target directly below the kobold. Hit: 6 (1d6 + 3)
bludgeoning damage.
You'll also note that giants, which are all Huge creatures, deal the same base damage with a rock sized for a Huge creature- 4d10. Treants, also Huge creatures, use the same base damage for a rock. This would indicate the damage dealt by the rock's weight itself is 4d10 damage (the varied Strength bonuses accounts for the force used to hurl the rock). Last, there is certainly no rule that states that any weapon, improvised or otherwise, deals damage based on how far it falls first. That would include a body... like, say, a bear. We can thus conclude that it is the size of the object falling that is the determining factor. This is confirmed by the collapsing roof trap- the amount of damage given is not dependent on how far the roof falls before hitting the character, but how much of it will (with the assumption being roughly a 5'x5' section of roof).
If we're treating falling objects as weapons, scaling damage becomes pretty straightforward. The standard rule is that a weapon gains a die of damage with every size category increase, and likewise loses a die of damage for each size category it drops by. At this point a little guesswork is necessary, as it is never explicitly stated what size category those rocks hurled by Huge creatures is in, but given the assumptions made so far and previous editions, a safe bet would be Medium.
Our makeshift damage table would thus look something like this:
\begin{array}{ll}
\rlap{\text{Falling Object Damage}} \\
\\
\text{Size} & \text{Damage} \\
\hline
\text{Tiny} & 2\text{d}10 \\
\text{Small} & 3\text{d}10 \\
\text{Medium} & 4\text{d}10 \\
\text{Large} & 5\text{d}10 \\
\text{Huge} & 6\text{d}10 \\
\text{Gargantuan} & 7\text{d}10 \\
\end{array}
Is this table perfect? No. Per this table, rocks dropped by Medium and smaller creatures should be in the range of 11 (2d10) damage, and instead we see about 6 for the winged kobold. If we call for a Dexterity saving throw for half damage, however, the damage output falls nicely in line. Using the collapsing roof as our basis, the DC for a falling object powered by gravity would be 15.
In my home campaign I'm slightly lazier than than this, opting for a 2/4/6/8d10 rule — but this is the table I came up with after the first time a character tried to use themselves as a missile weapon. I also have a standing rule that the listed damage only applies if the object/character has fallen at least 100 feet, so that for example the crazy gnome warlock that keeps jumping off his flying horse would deal 4d10 extra damage (lazy table version) if he hits, but take 10d6 damage doing so.
