Of course with non-humans there are no standardised sizes, but they can vary per setting. For example, in Middle-Earth elves are tall and orcs are short, where as in Glorantha elves are short and in Warhammer world orcs are tall and bulky. So you have quite some freedom in choosing how you want them to look like.
However, since most of the WF historical miniatures are perfectly useable for humans on a fantasy setting (or possibly even for other purposes with head swaps and conversions), it would be a pity if a possible fantasy range would not be compatible with those. Also, from a customer point of view it is most convenient when a given manufacturers models are consistently sized across the range, instead of being all over the place.
Size gets more important with fantasy humans (amazons, atlanteans), creatures that are partly human (centaurs), creatures that are man-sized (satyrs), or sets where you want to be able to re-use some of the existing sprues (eg. horses for cavalry).