For some reason, the fn:sum method sometimes returns a double instead of an integer, even when its arguments do look like integers. The problem is that doubles in XSLT2 are formatted using the scientific notation.

Thus, big image sizes may be reported in the completed moki file as:
 size="2.260045E6" instead of size="2260045"
This is in turn wouldn't be a problem if XSLT2 knew how to parse and convert such numbers into integers, but it doesn't work, and xs:integer('2.260045E6') results in a FORG0001 error.