If I am reading this correctly you are saying FF is better because it is larger than APS, therefor has less enlarging to reach a given size like 35mm against 6X7 ?
That's not the case, pixels are pixels. An 18MP FF image has just the same "enlargement" as a 18MP APS-C or DX or whatever. The advantage ( at least in theory) is that in the FF the pixels are larger and therefor should be able to record better information. However this advantage is being eroded with new technology. ( I know there are many other advantages, but "size" wise MP is MP.)
So, if you have a 18 MP iphone camera (assuming it existed), that will give you the same enlargement ratio as a 18MP APS-C? The enlargement ratio is based on final print size/sensor size. To cope with the higher enlargement ratio, the smaller sensor needs to have a higher resolution/precision lens to compensate for the smaller sensor.