In the Freetype backend FontCache::createFontPlatformData the pixel size is requested rounded to pixels, even though freetype supports returning a font scaled to a double value here. Also, the computedPixelSize() value is used as initializer for FontPlatformData's m_size, we should use computedSize() in both cases to allow uniformly scaled fonts without rounding.
Won't fix for now, since Cairo does not support subpixel font sizes.