This puts me in mind of a slender mouse boy impregnated by a milfy mare, resulting in him growing to the point of immobility with a whole litter of BIG babies… and then, after he's pushed out all six or seven of them and everybody's wrapped up, his girlfriend teasingly notes she wonders how big he'll get the next time.
…Or maybe a mouse impregnated by his elephant girlfriend, and everybody's wondering just what the heck is growing in that massive stomach…
…Ah, hells. Would you be uncomfortable with me writing something along these lines, presuming female, herm or shemale "daddy" and similarly gendered offspring? Any preferences as to which one? Or other little details you would want included in them - for example, the mouse/horse one is a "delinquent" student being impregnated by his roguish female (or "female") teacher?