Why insertion throughput can be reduced with an increase of batch size?

I recently faced with the observation which I can not explain. Why insertion throughput can be reduced with an increase of batch size?

Brief description of the experiment.

• PostgreSQL 9.5.4 as server
  
• https://github.com/sfackler/rust-postgres library as client driver
• one relation with two indices (scheme in attach)

Experiment steps:

• populate DB with 259200000 random records
• start insertion for 60 seconds with one client thread and batch size = m
• record insertions per second (ips) in clients code

Plot median ips from m for m in [2^0, 2^1, ..., 2^15] (in attachment).