While searching for the reason of QTBUG-17628 (huge fonts in some cases), I found that QS60PixmapData::metric(QPaintDevice::PdmDpiY) always returns MAXINT.
The reason seems to be that QS60PixmapData::cfbsBitmap.SizeInTwips() returns 0 which gets divided by another value to get the dpi, resulting in 0/x == MAXINT. The dpi value on an XM5800 should be 231, instead if MAXINT(2147483647). Huge Dpi's lead to huge fonts, if the font size is defined in points (QTBUG-17628).

QImage::logicalDpiY() always returns the correct value.
Edit: This only happens with graphicssystem "raster". So, on Symbian^3 it only happens when forcing the usage of raster.

See the snippet (output of the attached test program):

QImage im(100, 100);
im.logicalDpiX(); // 231
im.logicalDpiY(); // 231

QPixmap pm(100, 100);
pm.logicalDpiX(); // 2147483647
pm.logicalDpiY(); // 2147483647

Btw.: A very esoteric fact is that 2^31 - 1 == 2147483647 (231 == dpi on XM5800). So, the returned dpi by QPixmap::logicalDpiY() is somehow related to the expected 231