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These spaces comes because of responsive nature of modern pages. See the below link for more clarification:
https://sharepoint.stackexchange.com/questions/298725/remove-white-space-from-left-and-right-inside-our-sharepoint-modern-home-page
You can do this with some customization like:
https://support.shortpoint.com/support/solutions/articles/1000247707-how-to-remove-the-empty-white-space-at-the-bottom-of-sharepoint-page