Seems like insurers should be rating based on your worst health markers, including weight, over the last N years rather than just a current point-in-time snapshot. Someone who somehow has no medical records over the last few years at all that would capture any of that data would be priced on the assumption the past was possibly worse than current.
I don't know the situation in the USA, but in Europe you wouldn't find many young (up to ~35) people who have data on any health markers. And these are the main market.
