Couchbase document default size

I am saving json in the couchbase bucket. My json is of 300 bytes in size. However when i store them to couchbase, I see the disk size for that bucket to increase by 4kb per document.
I understand that database stores some metadata and indexes, however my document size is too small compared to the increase in disk size.
Is there a default disk space couchbase reserves for each document? Can we reduce this default size?