Has anyone any idea if there is any coloration between fat cover on cows/ewes and gestation period ?

A fat cow does not neseceraliy give a big calve but they defenitly have a harder time calving

A leaner cow will often give a bigger stronger calve and will calve often without assistance and will be able to rear the calve better

Fat cows generally give more problems at calving due to there being more fat around the pelvis. Fat cows are also more prone to metabolic disorders as well which again will cause calving problems. Calves grow the most in the last 6-8 weeks of pregnancy, so if you overfeed your dry cows you are likely to have bigger calves. Feeding the correct minerals is also very important, if a cow has insufficient magnesium in her dry cow diet this can lead to calving difficulties. I don't know what uni you went to or who your lecturer was but I would say that he doesn't know an awful lot about feeding cows!

 I've never heard of a coralation between fat cows and gestation period though.