How SQL server database reorganize memory
SQL Server does its own database file management - it does grow your .mdf file every time you need a few bytes more space.

Instead, for each database file, you've defined an initial size and how it will grow - by percent or by a fixed size. You can see these values by issuing this query:

SELECT 
    name, physical_name,
    size, max_size, growth, is_percent_growth
FROM sys.database_files


If your database file is set to e.g. 180 MB in size, and the growth is set to 50 MB, then as soon as the space in the allocated 180 MB is no longer sufficient, the file will be grown by 50 MB (to 230 MB).

By default, SQL Server also never shrinks data files - what for? Just to grow them again later on? It keeps track of unused space inside the file, and will reuse that space, until again, the file size isn't sufficient anymore, at which point it will be grown again (by that settings that's defined per file).