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Assuming both have the same sized pixels and same lens the full frame sees a wider field of view and thus more light. Not assuming any difference in QE, noise and certainly no extra light for each pixel of the same size between the 2 sensors. There may be a connection with less photon noise as the larger area can capture more. Not 100% sure of that though as generally it simply means a wider field of view when using a full frame versus an APS, hence more light.
There has been a lot of discussion of this aspect on DPReview in the past. Especially in the Fujifilm forum as its an APSc based system and it promotes the marketing claim that its system matches full frame performance.
In practice the basic gain of a full frame sensor may be diminished by higher QE, lower noise of well designed APSc sensors. Although in Fuji's case as they use Sony sensors with a different colour filter array the performance difference is questionable.
For nightscapes I see a big difference in sensitivity of my Fuji APSc low noise images and full frame either from a Nikon D800e (since sold) or Sony A7r. Also complicated by the fact that because Fuji has its own filter array normal ISO calculations seem to be twisted and ISO3200 on a Fuji X camera is not ISO3200 in brightness of a Canon, Nikon or Sony. Sony in particular seems very bright for the same ISO of other cameras I have used.
In practical terms you can see this at work in the nightscape section here. The full frame images are generally much more pleasing,brighter, wider field of view and lower noise than the APS images. So at the end of the day generally speaking full frame dominates in the Nightscapes department. Probably also with DSLR images with a telescope.
Greg.
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