Divide the objective lens size by the magnification to determine the exit pupil. A bigger exit pupil is more light being brought to the eye. All else being equal (quality of glass, coatings) a scope or binos with a bigger exit pupil will be better in low light than one with a smaller exit pupil (up to a point), but if there isn't much difference in this size the quality of glass and/or coatings could allow the smaller one to equal or surpass the lower quality optic.
I said above "up to a point". That point is probably 7mm which is the typical maximum dilation of the human pupil adjusted to darkness. That is why 7x50, 8x56, 9x63 are often the spec for "night" glasses, especially 7x50 because most people can't hold a binocular steadily enough for prolonged viewing with more power - their hands shake and with higher magnification it becomes more apparent. Magnification over 7x is more popular now, partly because most people don't know this rule and 8x sounds better than 7x, and 10x or even 12x even better. With modern lighter materials the shake probably isn't as bad and you can get away with something over 7x, but how much more depends on the user and the quality of the equipment.
But for brightness also note that as we get older our pupils don't dilate as much. E.g. for most of us in our 40s 5mm will be the maximum that our pupils dilate to so although a 7x50 bino is actually brighter, we older guys won't see it as any brighter than a 7x35 and might as well have the benefit of the more compact instrument.
I have a couple of older 7x50mm Bushnell porro prism models I like well enough but they are bulky and pretty heavy in the case of one, a rubber armoured marine model with built in compass, so I normally use a Minox roof prism 6.5x32mm which is lighter, more compact, and has better glass and coatings than either of them. I'm 49 and have seen other indicators that my eyes aren't what they used to be.