Size in bytes is significantly less ( half the size) then size in MB in get info window
387,037,139 bytes is the sum of the file lengths in bytes. A file however uses whole blocks on disk, hence even a one byte file takes one block. On the partition on which I tested this, a block is 4K. The “on disk” value is the number of blocks times the block size.

The difference between 684 and 716.9 is a factor of 1.024*1,024, that is the difference between the two possible meanings of megabyte.