Basically the racial feat never sat right with me in any version
The balance just seemed to be off at some point in level progression.

I figured I would make it a random roll to determine the outcome.

Anyway here is what I have come up with:

Fury of the Small:

When you damage a creature with an attack or a spell and the creature's size is larger than yours, you can cause the attack or spell to deal extra damage to the creature.
The extra damage is a die roll of twice your proficiency bonus. *Examples: Prof. of 2 = Roll 1d4,    Prof. of 3 = Roll 1d6,      Prof. of 4 = Roll 1d8,    Prof. of 5 = Roll 1d10,     Prof. of 6 = Roll 1d12.*
You can use this trait a number of times equal to your proficiency bonus, regaining all expended uses when you finish a long rest, and you can use it no more than once per turn.

Over all I like the idea, but it was hard to understand at first and took me a couple reads through to pick up what you were puttin’ down. You could make the language simpler and more like the way the official stuff is written if you model it after the way mind sliver and the Psi Die from the Soul Knife and Psi Warrior are written, kinda like this:

Fury of the Small:

When you damage a creature with an attack or a spell and the creature's size is larger than yours, you can cause the attack or spell to deal an extra 1d4 damage to the creature. This extra damage die increments one die size when you reach higher levels: 5th (1d6), 9th (1d8), 13th (1d10), and 17th level (1d12).

You can use this trait a number of times equal to your proficiency bonus, regaining all expended uses when you finish a long rest, and you can use it no more than once per turn.

It’s functionally identical this way, just easier to read and more familiar IMO. I hope that helps.