Pink seems smaller than Blue and Yellow so I was thinking maybe she's their little sister or something...but then I realized that they're gems so they can't have moms or dads or sisters. So was she like the "runt" of the Diamond family (I say family as an informal term; I just mean they're close) or was she created by one or more of the other Diamonds as a sort of Diamond Child for their amusement? Also, White Diamond looks like the biggest/tallest so maybe she's the oldest or something?
well, she could be off colored and came out wrong. More and more theories have out that white diamond created the other gems. That she is actually homeworld. She is seen holding homeworld. That the planet itself is white diamond
Maybe White is still building her colony, or maybe she was even shattered long ago and Yellow and Blue were so sad over Pink's "shattering" because they lost half of their family. More theories are that Pearl (the Crystal Gem Pearl) or Onion are actually White Diamond.
I honestly think that Pink is smaller is because to align better for the Great Diamond Authority symbol. Since it was shown of the outline in an episode that White is taller than yellow and blue because the white diamond symbol is placed above yellow and blue. Yellow and Blue somewhat close to each other of heights, Yellow is slightly taller than Blue. For Pink, her diamond symbol is below the yellow and blue diamond symbols. So, not only would the diamond symbol placement displays how much power a diamond has, it could also mean height-wise as well.
What I think is that, it's all about power/colonies/authority and knowledge.
When a Diamond has so many colonies and her authority is high enough (the case of White Diamond), they grow up and become taller. Since Yellow and Blue Diamond are roughly the same height, they both are important, but less than White Diamond (even though they all are important.)
If a Diamond that only has one colony (the case of Pink Diamond), their importance doesn't decrease, but they're still young and know less than the others who seem to know more about their colonies, and power.
If Pink Diamond started checking the galaxy (or sending her selected-troops), and make more colonies, then she'd probably be taller than what she is now, or simply because she doesn't want to.