The general assumption for lightweight heapdhones is that they don't seem capable of producing the same levels of sounds as the heavy ones. We'd say don't be fooled by the size, check the specs to judge the sound quality that can be expected. Many brands offer lightweight headphones just to cater to those on the go, so that they can enjoy their music comfortably for longer periods at a stretch.
Earphones obviously weigh less that headphones and range from about 5- 30gms where as headphones start from 100 and go up to 400 gms.
Sound quality for every headphones will differ but you can only gauge the comfort after having put them on for ten minutes. A lightweight headphone is more portable and comfortable for longer periods as compared to heavy Pro DJ headphones.. Then again these might have soft comfortable earpads that don't exert too much pressure on your ears. Most high end phones will come with leatherette earpads that are extremely comfortable and unlike foam pads dont go through any wear and tear.