aia file is a zip file in which all your app codes and assets are stored. You can share your whole project as aia. If you have created a new project then u can export it as aia and it can be imported on any account or other platforms. Aia file size is less as compared to apk.

AIA files are used to save backups of App Inventor projects and share them with other users. App Inventor allows you to drag and drop elements, such as buttons, images, text boxes, and sliders, to create a mobile application and program its functionality