I'm writing a story about the aftermath and consequences of a runic magic system being mysteriously introduced to the human race by an unknown entity. Every week a new rune is given to everyone on Earth over the age of 13, and everyone can keep any six of their choosing before having to give one up to make room for the next week's rune. Each rune represents a supernatural power the bearer is capable of using.
Initially society manages to adapt and cope with this radical change in the status quo, but by the end of year one, various factors, both from the rune system and from malicious third parties, combine to cause total societal collapse, resulting in a very long period of time where civilization is struggling to re-assert itself, and the superpowered survivors of the initial collapse and dieoff fight to survive in the ruins of the modern world.
This brings results in a certain issue I need to address. You see, during that first year when things seem like they're going to be okay, several one-week-only runes grant humanity the opportunity to take part in transhumanism. Occasionally a new fantasy race is introduced to the world, and everyone on earth will be given a totally genetically randomized body as a member of that fantasy race that they can switch between freely for that first week, and keep forever (in exchange for giving up their old body permanently) if they so choose by staying in that body when the rune disappears.
One of these races is a fairy-like creature called a lightwing. They're one foot tall, can fly, and while they're significantly weaker and more fragile than humans, it's not nearly to the degree that their size would suggest. You see, I want lightwings to be strong and tough enough that while they are weaker and more fragile than humans, their strengths and weaknesses with the other races balance out, and a fight between a lightwing and a human of equal skill and equipment would be a fair fight.
But here's the issue I'm having: If an individual lightwing is a match for any other creature, then when the apocalypse hits and resources become scarce, the fact that lightwings are tiny (and thus require way less food, water and space to survive) would give them an insane advantage over the other survivors during the initial decades after society collapses. People would be fighting over scarce resources that would go a lot further for lightwings than for anyone else. I can't see any reason why they wouldn't utterly dominate the post-apocalyptic environment.
The supplies that would keep a small band of five human survivors alive would be enough to feed a veritable swarm of lightwings that those five survivors couldn't possibly hope to compete against. And then when people start re-establishing communities and growing crops, the lightwing population would explode (relatively speaking) from the small minority they were before the apocalypse to much larger populations than any post-apocalyptic human community would be able to feed. When it reached the point of open warfare between these communities, the lightwings communities would rip the competition to shreds through sheer force of numbers.
Now, I obviously don't want this, but I still don't want to make individual lightwings inferior to individual humans to balance out their superior numbers, because that would somewhat dehumanize them and make them way less interesting as individual characters, especially since I want the story to revolve around smaller groups rather than large-scale warfare, at least at first. My initial planned solution to this was to make lightwings require just as much food as any other humanoid despite their tiny size, but the problem with this is that the concept of a tiny humanoid consuming, digesting, and excreting several times its own body weight in food per day sounds like it's going to need some very careful magical handwaving to not seem utterly bizarre (and gross). I'm prepared to try and handle that if I have to, but it occurred to me that I might not have to. There might be some other option I have to make the lightwing "zerg rush" strategy less viable.
What else, besides not having enough supplies to feed them, would keep my small fairy creatures from assembling in larger numbers than humans with the same resources and attacking in swarms?