Actually I think the DPI for PDF generation should always 72 be, see webpage.cpp:
#define PHANTOMJS_PDF_DPI 72
and further down in renderPdf
printer.setResolution(PHANTOMJS_PDF_DPI);
now for renderImage though... there the DPI is probably the QWebKit default which just might be 110dpi on your machine... Anyhow, if that's an issue you should report it on the bug tracker I think.
Cheers