*I think that you guys may have tried to backup an app with a higher size than 100 Megabytes, you may have noticed that you will get an .apk file that is lower than 100 Megabytes even if this app consumes 1 gigabyte of your device storage.
*The reason of this, is that Google Play doesn't allow developers to upload an .apk file >100 Megabytes, so for the apps that is bigger in size, they have to develop some additional files with .obb extension to contain all of the app data.
-Backing up these apps:
#Head to the following directory in your mobile: /sdcard/Android/obb
#You will find a folder named as following in general: com."developer name"."app name" with no spaces.
#Locate the file of the chosen app then open it you will find an .obb files/file.
#Copy all the files inside there to your computer or any safe place, you may copy the whole folder.
#Get any app that could backup your installed apps into an .apk file, for example ES File Explorer from Google Play.
#Backup the required app and you're done.
#When you are ready to restore the app, just create a folder inside the folder "obb" with the same name then put the files you copied from there before, or simply copy the whole folder again.
#Install the .apk file you have created through the app installer normally, then open the app while you have an Internet connection just to verify the setup and the app will automatically link itself to the folder you created.
Now your app is ready again