Why is the memory of my micro SD card smaller after being formatted through the app?

A part of the micro SD card is hidden partitions that are used for system files. Hidden partitions ensure the isolated storage and security of system files, and help with the high read-write efficiency of your storage card. 

 Therefore, it shows a smaller free capacity than its actual size after using the app to format the micro SD card.

If you want to use the entire capacity of your micro SD card, you can reformat the card on a computer with disk management tools.