It's so dependent on how the clothes are made, your height, how your weight is distributed, etc. I'm 5 feet tall, and at 331 pounds I wore a 32/34. Lost 15 pounds and was down to a 28. Lost another 15 and am still at a 28, but clothes are looser than they were.
When I was a teenager, I was 125 pounds and wore a size 8. My mom at 5 foot 6 inches was 190 pounds and also wore a size 8. We shared clothes all the time.
In short, sizes are weird!