Or you could take less biological approach and just call it magic :P. Another potential is that the 'kick start' needed to create new succubi/incubi they can get without feeding whatsoever (think of it as internal battery which was overcharged) which lasts until they reach sexual maturity age. So to get child born; loads of feeding (overfeeding) is needed from the mother (and with incubi partner it's easier as they can take some feed from incubi who itself could overfeed as well).<br/><br/>So far as I know succubi/incubi do not need food for nutrition but more for tasting things. Also breasts may serve nothing more than as means to get sexual energy from others (than store anything). Or breast size may indicate a innate potential for magic if we go silly but I think we overthink things too much :P.