Dgraph will do it’s best to rebalance the predicates (every --rebalance-interval) based on data size. The data and indices for a predicate would be part of the same Alpha group. Indices are stored on disk, so you’ll see more disk usage with indices.
You can check Zero’s /state to see which predicates are part of a particular group: https://docs.dgraph.io/deploy/#more-about-state-endpoint