One of my students has exported several .article files from InDesign. The example file is 3.3MB. She navigates to the article in AEM Mobile and 'replaces' the content with the file. It confirms the file is 3.3MB. If she then clicks to replace the file again, the sizes shoots up to over 20MB. She is concerned slow loading will affect her mark and neither of us can work out why it increases after uploading. When checking in Preflight, the article loads reasonably fast. I have a screen recording if this is of use.
I wouldn't be too concerned if it loads fast in preflight. The difference you are seeing can be attributed to compressed vs. uncompressed state. i.e. the .article file is a compressed zip file of the InDesign doc and resources etc. Once it is uploaded into the portal it is expanded.