Original color images size were 2M to 6M.. they were various.. However, when I transform into the B/w images, the size was decreased.. ;) So, now most of images are 1.5MB but dimensions are still around 4000 by 5000 pixels.
As you suggested, I have applied imresize() function to reduce the dimension so, I can handle in a pixel level. thank you for your suggestions. ^^