Well, where to start. A heifer that could not birth a goat, would indeed be a small heifer. That's not our problem. Please don't be offended by my remarks, but my son is 5th generation rancher, we've survived during the '78 foreclosures that many went thru- we've been here 'forever' and know the disposition and background of every cow, and during calving they are in close (but not barn confined) and checked, fed, and made sure general health is in top condition. We have discussed this with our vets, trying to see if there is something that can be done, or if it's 'just the way it is'. One thing that was brought to our attention; if the sacks could possibly be stronger due to excellent nutrition value of the cows, (say good protein, mineral program) that would explain why some people have no problems with sacks, the sacks are not 'strong' and tear easily. No, as far as we can tell, there really hasn't been any studies on such a thing.
Added: I am not saying if you have no problems with sacs, you aren't taking care of your animals with a good mineral program, etc. I could be the feed, who knows. One thing to add, our cattle are not excessively fat, but they are fed well.