Why is a .bacpac file so small compared to a .bak file of the same database?
A 


.bacpac file = Schema + Data. Note that Data is BCP'ed out using Native format (not readable by Human).


You can rename the .bacpac to .zip to see the actual contents. 
You can use ...DAC\bin\sqlpackage.exe commandline to extract the .bacpac contents programatically.
It is a snapshot that includes User data + Schema from SQL Server or Azure SQL Database.



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Backup (commonly have .bak extension) = A FULL database backup contains a complete copy of database and provides a single point-in-time to which the database can be restored. A full backup contains some transaction log records so that the restored component (database, file, or filegroup) can be made transactionally consistent.


A BACPAC is not a substitute for a FULL backup. Its just a snapshot that can be used to move a database from one server to another (or to cloud) and archiving an existing database in an open format.

From my test, below are the results