SQL Server Management Studio Doesn't do "Full" Backup
The issue is the defualt backup value of NOINIT
https://docs.microsoft.com/en-us/sql/t-sql/statements/backup-transact-sql?view=sql-server-2017

Media Set Options
These options operate on the media set as a whole.
{ NOINIT | INIT } Controls whether the backup operation appends to or overwrites the existing backup sets on the backup media. The default is to append to the most recent backup set on the media (NOINIT).

Go to the options tab and change the selection to overwrite, or change the SQL to INIT

Alternatively you can restore from the NOINIT backup using the directions at this post SQL Recover from .bak file with NOINIT