Wow, beautiful, but I don't want it. The weather I mean.
The heifers are fine. Sure, people refer to herefords and Charolais sometimes as pallet heads, but I really think that the shoulder size is more important than the head. When the calves comes out it can have any size head, but normally the shoulder is where the calf sticks (if it does at all). If you're concerned, breed them a low birthweight bull - which you normally do on heifers anyway. Since you don't seem to be a really big operation, you could also wait to breed them until they are a little larger - most people breed heifers when they are still growing, somewhere around 15 months.
Having said that, though, I really don't think you have anything to worry about. These would only be things that would increase your success chances. After a heifer has a first calf, assuming everything is ok, her chances of having calving difficulty are greatly reduced on the 2nd calf.
Just my :2cents: