There isn't always a correlation between the diameter of the optics and the light transmission. Some brands/models of 40mm will be brighter than another brand/model of 50mm. In addition, you also have to factor in the magnification. Generally, the greater the magnification the larger diameter you will need to get the same light transmission. That is why you can get by with a 2-6x32 when a 3-9 power scope would be a 40mm or a good 6-18 power would need a 50mm bell.
Just like everything else in the world, there are tradeoffs. The more magnification you have, the bigger the scope, the more weight you are carrying, the less the field of view, etc. My needs (hunting in FL with typical ranges less than 200 yds) is significantly different from the needs of a hunter in Wyoming (serious yardages in open territory). Figure out what your actual needs are and then limit your search to only those scopes which will meet those needs.