Ellie,
I'll take a little stab. Mtb hubs are built a little sturdier and heavier because they have to stand up to more punishment than road hubs are expected to recieve. I don't think there is much difference in the width of the hubs themselves. Dropout spacing for mtb rear wheel is generally 135 mm, road 130 mm so they use different axle lengths. However, I am pretty sure that one could use an mtb hub on a road bike wheel and simply replace the axle with an axle of appropriate length and adjust the spacing with different spacers between the cone and locknut to match the dropouts. Although one could do the same in reverse, using a road hub to build an mtb wheel, I don't think it would be a good idea because I doubt a road hub would last long under real mtb use.
Hope this helps.
Regards,
Raymond

Added - It was not my intent to repeat John E's message. It wasn't there when I started.