The same answer I gave above about pixel density and light receptors is still applicable to both answers.
If you google and read up on how sensors are made and how they work it will help.
I had a similar question a while back and a few people kind of got me started and then I read up on it until I was satisfied.
So in effect if you took say lens X and put it on a FF camera in the exact same conditions and f stop... then you put it on a crop camera in those exact same conditions... you do not gain 'total light'. The light density does not change. The T stop of the lens is the T stop of the lens.
Of course you have a bigger sensor but the overall light density is the same across the corresponding areas of both sensors.
The variable is the camera's sensor and it's ability of absorb the light at that density of light.
Long story short smaller light receptors that are scrunched in together make the sensor marginally less ISO sensitive than one of a larger size with bigger and un-scrunched light receptors.
It's not a sheer game of mega pixels. There are other factors involved.
Last edited by alamo5000; 10-12-2014 at 07:46 PM.