Most modern digital cameras produce very high resolution images, that are typically unnecessarily large. These take a lot of memory and can produce very large files. For example importing a typical 5 MByte digital camera image will add 5 MBytes to any saved .xar file. So importing 10 such images into a document could easily add 50 Mbytes to the file size.
When you import a photo, using drag 'n' drop from your File Explorer, or using the Import menu the program offers you the option to reduce the resolution.
If you select to have images reduced they are created at 1920 pixels wide (same as TV HD resolution) which is typically good enough for any screen and most print work. Selecting the 'Don't ask again' check-box will prevent this alert appearing again, and it will always do whichever option you choose from this point on.
Imported reduced size images will reduce the memory consumption.