Exports of "popular" reports, e.g. top countries or top pages, are limited to the top 1,000 items for the date range being viewed. If you wish to export more items than that, you will need to use the API.

Chronological reports, such as the visitor or action logs, can be fully exported, but only in batches of 1,000 at a time. If your visitor or action log export is more than 1,000 items, it will provide you with a link for each batch of 1,000.

The reason for this limit is to ensure large exports do not strain the database and affect other customers.