Hi @faheemrao1,
disk size is “Total disk file size consumed by the index”
data size is “Actual data size consumed by the index”
I don’t see any more detail in the documentation (https://developer.couchbase.com/documentation/server/4.6/monitoring/ui-monitoring-statistics.html) but I would speculate that it’s probably something to do with the size on disk vs how much of that is actual data. I.e. the data is fragmented to some extent. It might also have to do with indexes on non-data values (i.e. the document key). If you need more information, I can dig deeper.
As for using N1QL to get a list of indexes, all I know about is SELECT * FROM system:indexes, (documentation here: https://developer.couchbase.com/documentation/server/4.6/n1ql/n1ql-intro/sysinfo.html), but I don’t think that returns sizes. If you think that’s a necessary feature, I would be happy to create an enhancement ticket for you (or you could create one at https://issues.couchbase.com).