Using Outlook for iOS for personal email and calendar management
Power consumption can vary considerably depending on factors such as the number of email accounts configured, how frequently messages are synchronized, notifications, background activity, calendar and contact synchronization, attachments, network conditions, and how much each application is actively used.
Therefore, Outlook may consume more power than Apple Mail in a particular configuration, while on another device or with different usage, the difference may be small or even reversed. The battery statistics from your own device are more useful than a general statement that one application always consumes more power.
If you are experiencing unusually high battery consumption specifically from Outlook, please let me know approximately what percentage of battery Outlook is showing and whether most of the activity is listed as background activity, to check whether there may be a synchronization or configuration issue.