I have seen digital cameras with different sizes , some of them are small and slim in size ,?

but some of them are colored black and very high in size with same mega pixels why is this change , what is great of the cameras with high size and avg pixels like 7 mpa. do they they have any special features like high zoom or like that even their costs also a bit high that ordinary ones

Usually your smaller digital cameras have smaller sensors and use smaller lenses. They are typically called point and shoot cameras.

I am assuming that by larger digital cameras (colored black and very high in size) you are talking about DSLR (Digital Single Lens Reflex) cameras.

A DSLR will have a larger sensor, some have what is called a full frame 35mm sensor. You will typically get a better image with a DSLR because by having a larger sensor, you're recording more pictoral information. DSLR also have interchangeable lenses, so you also have much more flexibility in different shooting situations.

DSLR's also have more features. Shooting modes, white balance setting, greater EV compensation, great ISO sensor adjustment, etc.

DSLR's are also more expensive.

Larger lenses permit the use of larger CCD or CMOS sensors. Larger sensors give more detail resulting in better sharpness and color rendition.

yes, bigger cameras normally have bigger zoom, bigger sensor or more functions