They are loose.
But there is a reason.
You need to be able to move, without stressing them.
IF YOU PUT STRESS ON THEM, THEY WILL RIP APART
read that again.
I am 5-9, 155 lbs , I use medium.
The small jacket MIGHT work, the small pants will be both too tight and too short to hike in most likely.
My son is 5-8, 150 lbs, and I just got him a medium, he had a small.
I , intend to wear everything I have under it, and sleep in it when needed. So fleece and puffy have to fit, and still be able to move without stressing it.
Best thing you can do is order both sizes, and then decide.
Here is a pic of me wearing medium jacket, at clingmans dome on the AT, I weighed 159 lbs :