Under bright light your pupil may constrict down to 2mm, causing some of the light that emanates from a 5.25mm exit pupil to go unused because it’s blocked by the iris (above left in diagram). However, under dim light (like at dawn or dusk) your pupil will dilate up to 5mm or more (above right). Thus, an 8x42 binocular will deliver the maximum amount of light that your pupillary opening will accept. Next, contrast this with a 10x21 compact binocular rendering an exit pupil of 2mm:
Under bright light the compact will probably seem about as bright as an 8x42 binocular because the pupil is constricted down to 2mm (above left in diagram). However, because the pupil will dilate under dim light, a compact’s more narrow exit pupil will substantially diminish the brightness of the view (above right). This is one of the primary reasons most birders choose an 8x42; it’s the best combination of magnification and aperture, keeps the binocular’s physical size from being unwieldy, and still delivers a bright image at moderate power.