For me, it is a balance between size and optical quality. The lighter the binoculars are, the more I have them on me for hiking/ camping/ sightseeing. I have large high quality Pentax Binoculars (8x32) but they are heavy and I rarely take them on multi-day hikes. I now carry light weight, non-waterproof Nikons (8x23). They are light and have reasonable good optics/ brightness. And most importantly, they are always with me for identifying bird and mammals at a distance.
Peter