Same problem here! I recently bought one off amazon that fits nicely. I’m 5’1” 95lbs... it’s still a little loose but doesn’t hang past my butt like the normal “small”

JKSafety Class 2 High Visibility Zipper Front Kids Safety Vest With Reflective Strips, Yellow Meets ANSI/ISEA Standards (Kid-Medium) https://www.amazon.com/dp/B078LWTM8V/ref=cm_sw_r_cp_api_eevCBb9W35KAP