Sizing Index Service Nodes calculation query

Referring to section "Sizing Index Service Nodes" (https://docs.couchbase.com/server/current/install/sizing-general.html#sizing-index-service-nodes)

Doc ID = 20 bytes
Secondary Index = 24 bytes
Number of docs to be indexes = 20M

For the above, the page says that the index size will be 6 GB. How?

Thanks