First of all, make sure your application isn't hardcoding any font sizes. This is unfortunately easy to do in Designer, because if you change any font property (such as bold), then Designer will generate a UI file with ALL font properties set.

Otherwise Qt should be using the system defined application font.

Second, make sure other (non-browser) applications are reflecting this font size change. For some fonts and display resolutions, setting "Large Fonts" won't result in a noticable size difference. On my laptop, setting "Large Fonts" will make the titlebar fonts larger, but everything else will visually look the same.