The current version of Thunar presents the file size with a decimal prefix:
233,046,016 bytes are presented like 233 MB. This presentation is hard-coded and cannot be changed.

The binary prefix presentation of 233,046,016 bytes is 223 MiB.

I've witnessed a lot of confusion, and I think the best way to solve this problem is by giving the freedom to choose. I personally prefer the binary prefix, because the system displays it in this format.

You should consider adding a custom preference to avoid unnecessary confusion.