Function needs_size_check 

pub fn needs_size_check(
    last_measured_bytes: usize,
    delta_bytes_since: usize,
) -> bool

Whether the document is close enough to the warning threshold that its true compacted size has to be measured before the next commit is accepted.

Measuring means encoding a shallow snapshot, which is far too expensive to do on every commit. Estimating from the last real measurement plus the bytes written since it is always an over-estimate of the compacted size (history compacts, it does not expand), so a document can never cross the cap without this answering true first. That is the property that keeps the check from lagging the truth by more than a single flush.