The YXDB file is compressed which makes it smaller. That compression is part of what makes it so you can't append to one. The actual compression will vary based on what data you write to it. SQLite is appendable and queryable which inherently would be very difficult or impossible to support compression.
I was using a local mysql instance the last few days so I could insert 200k parsed xml files...1 at a time. It was way to slow doing this with read-union-write yxdb method, so I started using mysql and hated the fact I needed to add another technology to the stack.
This will work great to build my data and then export as a yxdb for faster down stream processing for other processes.