Crawling on stacked snow makes you half block tall but you can't fit in other half block spaces.

You can fit in a 1x0.5 space if you are crawling on a stack of 4 snow "slabs". Then if you try to move to other 1x0.5 space made out of a normal slab, you will not be able to fit there even though both the snow and the slab have the same hight. It also works when you make a full block of stacked snow and crawl under a slab.