Why is there such an enormous drop in bandwidth as the file gets bigger? Worse cache coherence? Youd expect the overhead of not having to initiate TCP handshak

Why is there such an enormous drop in bandwidth as the file gets bigger? Worse cache coherence? Youd expect the overhead of not having to initiate TCP handshakes, etc... to mean less overhead for a bigger request. Maybe Wrk is keeping connections open which reduces overhead in lower filesize conditions?