I don't know about the exact ratio, but kids that are light can get away with not being super strong for quite a bit longer than the bigger kids. I've seen kids that are much bigger than the average gymnast, several of whom would probably be medically classified as overweight, do well in gymnastics as long as they are strong. But a bigger, older gymnast that is also weak in the upper body is going to struggle.
Personally, I started gymnastics at 10. I was probably 75% in both height and weight at that age. I was very strong in the legs but not so much in the arms. I was able to progress to L8 but bars was always, always, always a struggle. I could have done at least Level 9, possibly 10 (at that time), on my other events but even at L8, my bars were not good for that level. My meets usually consisted of three 9s and a 7. :-/ It's not impossible, it's just much, much harder.