According to the official documentation: "A single call to BatchWriteItem can write up to 16 MB of data, which can comprise as many as 25 put or delete requests. Individual items to be written can be as large as 400 KB." (https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_BatchWriteItem.html)
But 25 put requests * 400KB per put request = 10MB. How then is the limit 16MB? Under what circumstances could the total ever exceed 10MB? Purely asking out of curiosity.
Actually I have also had the same doubt. Searched for this so much but found a decent explanation which I posted here (Don't know whether it is correct or not but I hope it gives you some intuition).
The 16MB limit applies to the request size - ie, the raw data going over the network. Can be quite different from what is actually stored and metered as throughput. I was able to hit this 16MB request size cap with a BatchWriteItem containing 25 PutItems of around 224kB
Also once head over to this link. This might help.