If you want to compare load though you also have to consider all use cases. You might need to know the username for each follower, and thus have an extra API request, but there are others where that data would be redundant, so adding an extra field (or multiple fields, as who is to say some users might want display name rather than username) would be needless data for those people, data which would be on every single request regardless of if it’s needed or not.
Only twitch have the data to know what impact additional fields would have, and an idea of how frequently used those fields are.