I have heard that leaving a small amount of food is considered respectful, why is this?

I do not hear about such kind of custom in China. As far as I know, leaving nothing in the bowl shows the respect to the host.

Generally, when we have a family party or some other parties, Chinese people often leave some food means the host has offer enough food whilealso they are very delious,

Leaving a small amount of food on some special wealth dish is considered respectful and prosperity. When you are eating a dish which celebrates new year's wealth (sorry I can't remember the name) is good to leave some on the plate so as to not finish all your "gold" and leave some behind for next year.

When I was small, I was told like if u don't finish the food in your bowl, u will married to an ugly guy

As far as I know, You shouldn't be leaving anything behind but sometimes... Ok I don't really know but I heard my grandfather say that ppl leave it behind for the ghosts to come and eat. I really don't know....

I know what it is. All of the east asian culture erea's country have this culture including China Korea and Japan. In east asian culture, they believe the food including everything just like a space, or water, humanity, nature, the passion of the farmer. So leaving with empty bowl isn't mean just respect to host. It means that respect all of the space including host