When using lowercase search and replace terms with :s, the casing will intelligent replace what the search term is currently using. To enable this, both terms in the action should be lowercase.
:%s/small/large/g will change both small -> large, and pxSmall -> pxLarge.
This is also helpful when changing multiple words on a single line in combination with visual block selection, e.g. :'<,'>s/small/large/g.