There’s nothing realistic about Abby’s physique considering the world she lives in:

1. The WLF rations food, so it’s impossible for her to hit the daily caloric needs to maintain her musculature.
2. Abby has trouble sleeping. You need good amounts of uninterrupted sleep to gain and maintain muscle.
3. On top of her bad sleeping schedule, she’s also spending every day doing strenuous activity as a result of exploring, gathering supplies, and beating enemies.
4. Having a bulky build would actually be detrimental to survival. Agility and endurance are far more important than muscle mass.