Photoshop :: Resizing Image - Look Pixelated

Why, when you downsample a picture  (say from 300 to 72 dpi) does it look look pixellated, even though it is scaled down in Indesign and the effective DPI is well over 300.

I am using little icons that are clipped out photos they will be printed at about a quarter inch. These icons are on a bunch of calendars so they are placed in Indesign hundreds of times. Their actual size was originally about 5 inches at 300 dpi -- wayyyy larger than necessary for the final printed size.
 
I need to downsize them, without have to adjust each one in the ID document as they are already placed. I can do this by downsampling them in PS, from 300 to 72 dpi and not change the dimensions.But when I do this, even though they are so small in the ID document, they still look pixelated.