The document of $wgMaxUploadSize (https://www.mediawiki.org/wiki/Manual:$wgMaxUploadSize) suggests it can be set to an array, thus offering the ability to vary url max upload size from direct max upload size.
In the page form upload window, however, if wgMaxUplaodSize was an array, a fatal error would be thrown by
wfMessage( 'upload-maxfilesize', $this->getLanguage()->formatSize( $wgMaxUploadSize ) )->parse() . ' ' . wfMessage( 'upload_source_url' )->escaped(),
since the message handler couldn't deal with the array parameter. This can be easily reproduced in Page Form master branch as of March 14th, 2017.