Please, can you better explain what you mean with the bolded following sentence taken from official documentation?
Your Piwik database size will grow over time, this is normal. Piwik will delete archives that were processed for incomplete periods (i.e. when you archived a week in the middle of this week), but will not delete other archives. This means that you will have archives for every day, every week, every month and every year in the MySQL tables. This ensures a very fast UI response and data access, but does require disk space. We’d love to see a a plugin that would delete some of the old data (for example, only keep the top 50 rows for each report).