While updating the App Icon, you may receive an error message File is too big (XX.XXMB). Max filesize: 4MB and the image won't be uploaded. The reason is related to the size of the file that you are uploading, it can't be greater than 4MB.
While following the steps as explained in the Changing The App Icon article, you notice that the image is not uploaded and the error message is displayed:
This is because you have exceeded the maximum allowed size.
You will need to use any image editor or online service that allows you to reduce the image file size to a value lower than 4MB.
Once you reduced the image file size and upload it again, following the steps provided in the Changing The App Icon article, you will receive a successful message: