I am 5' 1" and weigh about 107. My waist is small and my hips, while not big, are obviously bigger than my waist. I am not built like a boy... I bought a Nathan waist pack and it works really well for me. I went into the store saying, "I have a really small waist," thinking it would be hard to find something that didn't slide around. Boy, I was chagrined to find that the small sized Nathan pack was snug! I can adjust the velcro very comfortably.
Now, I only run short distances, but I get thirsty very easily and after about half a minute, I forget I am wearing it.
2015 Trek Silque SSL
Specialized Oura
2011 Guru Praemio
Specialized Oura
2017 Specialized Ariel Sport