Though, if Pink took on the form of a Quartz soldier, and her Quartz proportions were passed down to Steven, wouldn't that mean that a good chunk of his weight would be muscle? He has been shown to be ridiculously strong for his size.
At the risk of bringing reality into a discussion about a cartoon, BMI is a very flawed tool! It's designed for measuring trends in populations, not individuals. It may be a useful indicator sometimes in some circumstances, but other than that it's far better to just be active (which Steven is!) and try to avoid too much junk (which he certainly does not 🤣 though he's possibly improved his diet more recently!).