Laurent is right here. Databases experience different physical sizes on different platforms (file system, disk allocation size etc.) and even on identical platforms because of various reasons (database growth settings, order in which data was stored etc.). Actually it is just by chance two databases with identical content would have exactly same physical sizes. By physical size is understood the sizes you see at the file system level.
A different thing is the logical size of a database. In order to function efficiently a database system reserves disk space in advance and formats it suitable for the database's use. A healthy database should always contain such unused space.
To repeat what Laurent said
But anyway, if the data otherwise is valid it’s not a Joplin issue, but an SQLite one, and it’s most likely not a bug.
It is a database issue and it's not a bug.