Why is the dog in the sandlot movie big in the whole movie but at end it is normal size?

It is an illustration of how we picture things as kids and how our perceptions are often blurred by our emotions....notably fear.  When the dog is seen as his "normal" self at the end of the movie, it represents how our perceptions become more "real" when we overcome those fears.

This is a common device in movies featuring storylines where children must overcome their fears.  In "Home Alone," for example, young Kevin pictures the basement furnace as some ominous, fire-breathing monster to justify his fear of being alone in quiet sections of the house. 

By the movie's end, after his victory over the criminals, he realizes that his fears about the furnace and the "scary" neighbor were unfounded and sees both as "normal"...

Movie magic..................