I have built a file in Animate CC Version 2018 and exported it as an OAM file. The OAM I imported works like expected but the file size is 562,4 MB, so it is too large. Any published .fla file (small or large file size) the OAM file size is 562,4 MB. What is wrong?
oam is just a zip file. You could duplicate the file, rename it to .zip instead of .oam, then unzip it to see what it is that's taking up so much space.
It's possible that your images are all being published as PNG for example, or you have a lot of sounds and you're publishing them at high data rate.