If I remember well, you got sheep, right? (or do I mix you up with someone...)
Have you noticed lambing difficulties when putting a ram of a bigger breed on an ewe? Or is it something you expect?
In some species, the size of the baby at birth is defined by the mother (and the feed), while in others it is defined by the father as well. I sheep I don't know, and don't find much (don't know how to search).
Did someone hear of an accident where a big ram escaped and impregnated dwarf ewes?