Why is my disk size reduced after creating an ext4 partition?

Your SSD has 200 GiB (1024-based units!) of disk space total. After formatting with ext4, you have 196 GiB left; ext4 needs some disk space for its own meta data such as i-nodes and reserve blocks.

Those 196 GiB are the net capacity of that partition /dev/nvme1n1p1 mounted at /media/Data. As usual, some of that is reserved for the root user, leaving 185 GiB for normal users.

Reserving some percent for the root user makes sense if it's a vital system partition such as the root filesystem; but for a pure data disk like this it does not. Yet mkfs.ext4 does it because it cannot know what you intend to do with that filesystem.

The reason for reserving some disk space for root is to give the system administrator a fighting chance to work with filesystem tools if the disk fills up; if there are zero blocks left because some user filled a system partition completely, it's getting hard to do anything about it because some tools also need some disk space for normal operation.

But you can make those reserved percent available for normal users with the tune2fs command:

sudo tune2fs -r 0 /dev/nvme1n1p1

See also man tune2fs.