Large drop in performance of sync writes when increase document size

Summary

When performing sync writes the throughout drops dramatically as the size of the document increases.

Steps to reproduce

Use cluster run with 2 nodes (just KV service running)
create a single bucket - with default settings (i.e. one replica).
use cb-pillowfight from latest version of libcouchbase
run cb-pillowfight as follows for 10byte docs:

./cbc-pillowfight -U 127.0.0.1:9000/default -u Administrator -P asdasd -I 1000 -B 100 -t 1 -r 100 -m 10 -M 10 --durability=majority

On my MacBook Pro this achieves 700-800 ops/sec

Now trying 1K docs

./cbc-pillowfight -U 127.0.0.1:9000/default -u Administrator -P asdasd -I 1000 -B 100 -t 1 -r 100 -m 1000 -M 1000 --durability=majority

The throughput drops to ~100-200 ops/sec

Now trying with 5K docs

./cbc-pillowfight -U 127.0.0.1:9000/default -u Administrator -P asdasd -I 1000 -B 100 -t 1 -r 100 -m 5000 -M 5000 --durability=majority

The throughput drops to < 10 ops/sec, and cbc-pillowfight is starting to report timeouts.

Expection

My expectation is that the increase in document size would have significantly less impact on performance.