This is normal. Backups only backup the actual database. When the program is upgraded or things get deleted such as the tables (within the database), this does not reduce the file size of the accounting file, but when you backup the accounting file it will backup the database without all the deletions and tables/rows removed from the database over the years. Hence the backup file will always be smaller than the original file.
This is known as database shrinkage so is perfectly normal and to be expected.
If you are worried about it, you can rename the backup file and import it and you will see all your data is there with no loss of data.