Can you explain more what you are doing and seeing? What differences do you see? When you look in the folder, it's unclear what you are checking here. Is it the file size in bytes? Depending on what you are looking at, things might be different.
for example, sp_helpdb gives me a size of 80.00MB for my Sandbox db. In the file system, I see
- sandbox.mdf - 8,192kb
- sandbox_log.ldf - 73,728kb
That's 8192, and if I divide by 1024, I get the 80MB. You should be able to do something similar, but there might be some rounding difference, so this is why I'm curious for more details.