Too small because of underscan? I'm baffled why with HDMI and the 1080p standard it's now the other way round - I need to disable overscan to get the full image

Too small because of underscan? I'm baffled why with HDMI and the 1080p standard it's now the other way round - I need to disable overscan to get the full image and games still have to prompt me to choose a safe area to display UI elements.