You remember how in "Adventures in Light Distortion" when Steven is trying to get everyone to look like the right gem, well, all of the gem shapes they transform into are real, right? So that means that the extremely small gem is an actually gem. What gem do you think it would be?
Dude I have no idea but I am DYING to know. Also since we know all Gems are made for a specific purpose, then that means whatever the teeny gems are, they were made that small in order to be able to perform some specific function. Since we know Homeworld sees shapeshifting as “an insult to your intended form” (or something along those lines, quoting Peridot), it would make sense for them to have a small Gem to use for making precise repairs inside small, inaccessible parts of ships or something (rather than just having a Peridot or some other technician shrink down to do it).
On the other hand, Peridot said her size was indicative of her rarity and importance. So maybe with the exception of Diamonds, the smaller the Gem, the more important they are? Rubies are given more autonomy and responsibility than the Amethyst guards, for example (they were allowed to pilot their own ships and go on missions, whereas the Amethysts were just standing guard). Sapphire is the smallest Gem we know, and she is also very highly ranked. But then again, it seemed like Holly Blue Agate was higher ranked than Ruby and certainly Pearl, so there may be other exceptions. It also seemed like Pearls carry some degree of the rank of their owners–for example, Yellow Pearl could probably tell off one of the Amethysts and it would be seen as normal, because even though Pearls are objectively the lowest we know of in the Gem hierarchy, they inherit some authority from being associated with someone like a Diamond.