Only "I know how to change my own" is in the default size. The rest of the OP is in various custom sizes.
OP, that's going to be a browser-side or device-side setting.
If you're using a desktop browser, the first thing I would recommend is adjusting the browser zoom level for QQ -- try looking for a "zoom" option in the menus, or pressing crtl-+. You could also try overriding the browser's minimum font size, or doing custom CSS such as with an extension like Stylish, but those risk breaking how things look.
If you're using a mobile (phone or tablet) browser, try looking for something about text size under an accessibility menu. I was assuming you'd have to go to your device's settings, but I just found an accessibility menu with a text size option in my Firefox, so check your browser too.