The pupil is the opening to the eye through which light passes to project the image of the world on to the back of the eye which is lined with light sensitive nerves called the retina. We refer to these nerves in the  retina as rods and cones. Humans have both rods and cones allowing us to see both in colour and black and white. Dogs on the other hand only have the types of nerve receptor cells contained in their retina that allows them to see in black and white only. Cats are a little more lucky as they can see in colour like humans however cats are at a better advantage over humans as they have the advantage of having exceptional night vision and are able to see quite clearly even under very low lighting conditions .e.g. at night with no street lights on.

The pupil of the eye will dilate (open wider) under low lighting conditions in order to allow more light to pass into the eye for better vision. 

Also as a reflex reaction the pupils of the eyes  also dilate (open wider) when something  we perceive as visually stimulating enters our vision. It's also true that they will also dilate through arousal whether connected or unconnected to direct visual stimulus. Within the appropriate context with less possible number of external influencing factors as possible it could also be one of many indicators of attraction if you actually made the observation in the first place to notice the difference afterwards. Of course this would be very difficult to do if you're at a rave where everyone might be wired on Ecstasy since it obviously has the side effect of causing constant pupil dilation until the effects wear off.  

Flight or fight

The pupils of the eyes also dilate when we're placed in situations that increases the amount of adrenaline released into the body which extends from the flight or fight reflex/instinct.  

Obvious but evidently not to some

No human being is able to see like a cat, just as I'm sure the majority of people see in colour. Cats and dogs are not able to reproduce A sexually. There are also female dogs just as there are male cats.