Our average PutBatchRecord size is 1Ko bytes.

D, [2020-01-14T10:16:30.981466 #1] DEBUG -- : [Aws::Firehose::Client 200 0.033437 0 retries] put_record_batch(delivery_stream_name:"kxxxGe" ... (1429 bytes)>}])

Regarding to AWS FAQ:
https://aws.amazon.com/kinesis/data-firehose/faqs/#Pricing_and_billing

Q: When I use PutRecordBatch operation to send data to Amazon Kinesis Data Firehose, how is the 5KB roundup calculated?

The 5KB roundup is calculated at the record level rather than the API operation level. For example, if your PutRecordBatch call contains two 1KB records, the data volume from that call is metered as 10KB. (5KB per record)

We are not optimizing our costs.

I don't see which kinesis_firehose plugin option can help to increate PutRecordBatch size.

Regards,

JB