At 30lbs I think it's safe to say the large RSTV would be too loose. It was nearly too loose on my son (95th for height and weight) when I bought it for him and he was at least 40lbs then, actually I think he was 43lbs. I had it cinched as tight as it would go. You might see if you can find someone on here who has a small and could measure it from shoulder to the bottom of the vest. Or are there measurements online? I just loved the ease of traveling with it! They so need to come out with a medium. There is just a pretty large gray area that neither size will fit or fit well, at least.
He is 37 lbs. I was going by the measurements and description in the car seat data pages, which said that the small would require a chest circumference of at least 22" in order to tighten, the circumference of the vest being 24.5". His chest circumference is 21" with a thick shirt on. The length is listed as 16.5", and his torso seems to be about 17.5". So it sounds like even the small would be too big around, and also possibly too short. I definitely would get one though if I thought it would fit.
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