In one of my memories where that is the case, it has to do with my actual physical size. When I was 5 yrs. old and stood at the candy counter, the top of the counter was wayyyyyyyyyyyyyyyyy above me (at least 8 or 9 inches)...it was high....the merchant towered over it and looked down at my friends and me through the glass top case. Now as an adult, I can stand at the counter and place my hands on it so now it does not seem tall to me at all.
Because as children we are looking at the world throught those eyes (a child eye/viewpoint). Everything around children (except their friends, toys and playmates) are bigger, much bigger. As we grow everthing becomes proportional to our growth until we become adults. That is why the adults in a childs life should always make them feel safe and cared for.
Good question. We don't often think about things like this....
I have experienced this many times and could only attribute it to a spatial context of size. I visited my old middle school several years ago and was shocked at how small it seemed, the hallways seemed more narrow and the length of them was ridiculously shorter than I had remembered. So I am guessing that as smaller kids more of us fit between the walls than we would as adults.