The Nikon AF-S 50mm f1.4 G is not designed for the smaller DX sensor and has the same image circle as the 1.8
Are you thinking of the AF-S 35mm f/1.8 G DX which is designed for a smaller sensor compared to the AF-S 35mm f/1.4G?
In any case the size of the image circle is of minor importance to the light gathering ability - only the aperture matters, as Evan states.
Despite a lens designed for a smaller image circle letting in less light total, it lets in the same light per unit area, so if you swap one lens for another designed for a larger image circle, but the same f-stop then your exposure would be the same.
The previous paragraph ignores vignetting, or the tendency for brightness to fall off across the frame. This usually gives lenses designed for a larger image circle a brightness advantage. There are other advantages to using lenses designed for the sensor you are using, such as sharpness, weight and better resistance to flare.