By default Tixati uses decimal unit prefixes not binary prefixes. For example:
(decimal) 1 MB = 1000 kB = 1000000 B
(binary) 1 MiB = 1024 KiB = 1048576 B
As you can see "binary" MiB is 48576 bytes larger than "decimal" MB. To add confusion, almost everyone uses MB prefix but means binary MiB instead. If on a torrent site they have 700 MB movie, Tixati will display 734003 K by default. You can change prefixes in Settings > User Interface > Output Formatting. Change Largest Unit for Bytes and Bytes in Charts to Mi, you'll have the same units as on most torrent sites.
For more information read: http://en.wikipedia.org/wiki/Binary_prefix