I believe survival/apocalypse games are better suited to smaller groups, pockets of separated small groups, or need to have strong rules about danger/mortality the larger the group gets.
See, humans at one time farmed and had to set watches to keep wolves from eating people in huts, right? But the problem I see on most survival/zombie/apocalypse games is that character types are regularly chosen to fill niches until the characters themselves negate the survival aspect of the game. By this I mean: Is it REALLY a survival game when you have fifteen soldiers, a former Navy Seal, three doctors, one veterinarian, a brain surgeon, four people with agrarian science degrees, a botanist, three midwives, two stable masters, two blacksmiths, seven leatherworkers, fifteen carpenters, two daycare workers, a horse whisperer, two poli-sci majors, six former police officers, and one massage therapist AT THE START OF THE STORY????
Survival/apocalyptic stories are not about guy broke his leg, call the doctor, but instead they are about guy broke his leg and we don't have a doctor. They are typically about regular joes (or to quote All Flesh Must Be Eaten: Norms) and only a very few vital (maybe 5% of the population) people with very useful skills. The stories are about overcoming these adversities and surviving while unlearning old useless skills (JAVA PROGRAMMING!) and replacing them with new ones (EATING MUSHROOMS AND NOT DYING).
So the main issues I have seen are characters who are too useful, and not enough emphasis on how such large groups are able to survive without fatigue from lack of resources. With so many capable characters, the end result is like No Return Mu: "That sucked, oh well, let's just rebuild houses and start new hospitals! We have oncologists!"
TL;DR version: Big groups are okay so long as they're not filled with fucking experts in survival-useful skills, and so long as people realistically roleplay the struggles of survival, rather than hand-wave the actual survival part.
I don't play "100", but are there a lot of characters with super-useful skillets despite being teenagers?