Err... It isn't lens. It is sensor size you are talking about. Nokia 808 used 1/1.3" and Lumia 1020 used 1/1.5" sensors. And 1520 used 1/2.3" and 930/icon used 1/2.5".
The reason is the larger the sensor, the more space it occupies in term of thickness. That's why 808 and 1020 both have camera bumps. 1/2.3" or 1/2.5" sensors can be accommodated while keeping the profile thin without the noticeable cost in quality. This also helps keep the price in check.