If you want to know why, it can be explained pretty simple. As an image is build up by pixel.
So if you imagine having an image 500x500 pixel and you resize it, all the information in the image that it used 500x500 pixel to store before, now need to be stored in lets say 64x64 pixel, so you will loose a lot of details.
And the same is the other way around, if you use 64x64 pixel to store lets say a picture of a human face, and you resize it to 500x500 pixel, a lot of blur will occur, as it trying to fit very few information into a lot of information. So as a general rule in my opinion, its always best to create your graphic as close to what you need, and in case you cant, or need to resize it, its always best to shrink over enlarge.