TYPO3 Core

TYPO3 may hit memory limits when converting XML to PHP arrays, even before all PHP memory is consumed. This is probably due to a specific limit of 10 MB in libxml, which was introduced with libxml v2.7.0. This limit could be impractical when importing large XML files via EXT:impexp.

One solution could be to turn off the libxml limit, but this is only supported for the DOMDocument and simplexml_* implementations of libxml, but not for the current XmlParser implementation (a.k.a. xml_parser_*() functions):
https://stackoverflow.com/questions/5428055/xml-parse-huge-on-function-simplexml-load-string

Another approach could be to read and parse the XML file in blocks, as suggested in this stackoverflow thread:
https://stackoverflow.com/questions/19152613/xml-parse-no-memory-error-php