Ideas about really small calves????????

[RAWANGUS]

This is the second calf born under 40lbs.  Actually one is a Set Me Free X Unstoppable calf that weighed 35 lb.  The other is a I-80 X Jake's Proud Jazz calf weighing 39 lb.  All of the other calves this spring have been around 70-100 lb.  Mothers are fat, been out on grass the last month of pregnancy.  Vaccinated annually.  mineral blocks year round.  Never had this before.  The Unstoppable cow above had a first calf (Jesse James) that weighed 60 lb.  The Jake's Proud Jazz was a first timer.  Any ideas of what is going on.  the calves are alive, vibrant, running, eating, just look like little fawns.