[gernerttl]

Aperture on a camera is the ratio of the lens opening compared to the focal length. If the focal length of both phones is equal (which they are), the lower number will have a larger opening, meaning it will let in more light.

The larger sensor just means the sensor can capture more detail with the light provided to it. The opening in the icon is physically smaller, so it's letting in less light.

Lens opening or F-stop is ratio of the focal length/aperture in millimeters. Not the other way around. And yes, sensor size does make a difference. A smaller sensor gives you a lower effective F-stop than a larger sensor. The F-stop not only determines the amount of light the camera can gather, it also affects the depth of field. Smaller sensors do not gather light as efficiently as a larger sensor with the same resolution; this becomes a problem in low light situations.