You still have to read the stored size on each parent to figure this out, at which point the optimization makes sense only if writes are significantly (at least

You still have to read the stored size on each parent to figure this out, at which point the optimization makes sense only if writes are significantly (at least 2x) more expensive than reads, and this is not true for most desktop PCs. This really boils down to a caching problem, and well, there's a reason it's one of the two hard computer science problems..