The current limit is 20 MB.
Delete some unnecessary sound and picture files (or reduce colors in pictures and convert files to a new format to save space)
Try to figure out what is taking up the most space.
You might be able to procedurally generate some of it.
Thank you for replying guys. Fortunately, I could find some unused sound files in my game's folder. I removed them and scenario's size was significantly decreased (up to ~10-13 MB). Soon, I will post a new scenario, but its current situation is "Requires more analyses" - to figure out why after some time it freezes + some code refactoring.