Constant KEEP_BELOW 

const KEEP_BELOW: usize = _; // 65_536usize

The largest subject an entry is kept for, measured over the strings and record bodies it owns.

Reuse within one write happens whatever the size — the request is built once either way. This is about what is still held afterwards: a record at the 1 MiB an atproto record can reach, multiplied by the worker threads a write can land on, is real memory on a 2 GiB host. 64 KiB covers an ordinary record comfortably and bounds the rest.