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It is a bit of a comparison between a Nissan Micra and a Toyota Avensis. Different specs.
A 15x70 captures twice the light of a 7x50, and goes a lot deeper than a 7x50, but a narrower FOV. A 7x50 shows the whole Southern Cross in a single field (7º), while the 15x70 can just show Orion's belt (4º).
And most cheap binos have too small prisms so the aperture is capped: I had a 15x70 which was actually 65mm and an 10x50 which was actually 44mm.
You can check that by holding a flashlight (a cellphone's builtin flashlight can also be used) behind the center of the eyepiece and holding a flat surface (e.g. a white wall) a few cm in front of the objective which shows a circle with the diameter of the real aperture.
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