I believe this would also help my issue which is in the old MPS Atlas you could change the extent of the map frame based on the size of the index feature ie. a map index. This helps when your map index has different sized maps due to having 'special' sizes or when you are creating a map series for a whole country (AUS) when the size of your map increases the further away you get from the equator. eg. the map still covers 1 degree by 1.5 degrees but the size on paper is enlarged, therefore to show the whole map in your data frame it needs to enlarge.
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