To be clear here ... what you see outside is not the lens, its the lens cover (the fancy ring and transparent circular stuff) and not the housing....
the housing of the lens is on the pcb..
the actual lens assembly lies underneath the lens cover (the tiny blob).
I totally agree with the people who said "I would think its a design choice". Design in the sense external design. (looks of the phone)
It has nothing to do with the internals.
Now coming to the sensor size ....
Bigger the sensor better the quality. (Greater the megapixels, greater the quality is false).
Megapixels cramped into a small sensor is useless.
So it all comes down to sensor size.
Yes a 10MP on 1/2" is better than 10MP on 1/3"
To accommodate a bigger sensor and a bigger lens assembly you have camera-humps visible in most of the smart phones.