I have noticed that two project files that were created in S2 are much bigger size than the one I created in S3. All are text only, no images graphics or attachments.
One of the former S2 files is approximately 12 MB whereas the new S3;file is around 1MB. I appreciate it is difficult to draw comparison but I am wondering whether S3 is more ‘efficient’ in which case I am thinking of copying and pasting all the content from the S2 file and creating a new file for it in S3.
If anything it is the other way around by a small amount. Scrivener 3 has more redundancy and architecture inside the format, to better protect it against synchronisation issues.
Otherwise, it’s really not possible to speculate on what the difference might be without a better comparison. Are you saying that if you load a 12mb project and update it to v3 it drops to 1mb, or are you just talking about two entirely different projects that happen to be different sizes?
It may be easier to load the project in v3 and update it, instead of copy and paste. You might want to do that if for some reason you really don’t want any of your settings, labels or keywords, etc.