why Mens wrist watch is bigger whereas Womens wrist watch size is too small?

If you are having trouble finding one that fits, try an elastised stretchy band or one with adjustable velcro.

I think normally men are harder than women. Their wrist seems bigger than women. So your question is.

The market would sell larger wrist watches to women than men if women would buy them. Women usually have smaller fingers and wrists and want smaller rings and wrist watches to be feminine. Nothing prevents women from wearing men's watches except taste, especially because there are matching sets of men's and women's watches that differ in size but not in style. Women even wear the pants in some families .