You can select the size of the photo from the Settings screen in the app “Import > Image Size”.
This is the size setting for the photo cache data displayed on the photo detail screen in the app. Currently, the maximum size is 4096px, and if the original photo data is larger than this size, a cache file will be generated for viewing. (No matter which size is selected, the original size data is not lost.)
Basically, if you select the “Show Original”, no extra cache file will be generated and the app will use less space. If you have an older device and feel that the photo viewing screen is too slow, please select a smaller size.
If you have already imported photos, you can resize them as described above and then go to Settings > Import > Batch correction of Image Size in the app. This will also remove unnecessary thumbnail images, so you can reduce the amount of space used by the app. Please give it a try.