The antagonistic pleiotropy theory actually advocates exactly that inverse correlation, more investment into young years for increased reproductive fitness at the expense of worse later years.

Further, I suspect that number of kids or even just a binary having kids or not would have a direct effect on BMI in old age. More kids > less time to excercise > compounded over many years. There's also a known correlation between educational attainment and number of children in the literature which is generally hypothesized to be causal.

I have concerns about the validity exactly because of the expected tiny effects. Polygenic traits with extremely high heritability (height being the prime example) top out at 10% of variance explained even when considering all alleles. That just highlights the need for better phenotypes and multi generational typing. I think it's Botstein who I've heard advocates for more time points over more replication when given the choice.