Vitamins and supplements are essential for the pregnant mother. The World Health Organization recommends routine iron supplementation in pregnant mothers. Vitamins belong to a DIFFERENT CLASS of food altogether. There are carbohydrates, fat, protein, fibre, water and there are vitamins. Vitamins do not have CALORIES!
So, the common notion that 'taking vitamins during pregnancy makes my baby large' is illogical and not supported by evidence.
You may get a large baby if its in your genes, you develop diabetes in pregnancy, develop excessive weight gain due to excessive carbohydrate intake or the baby itself is abnormal. Your baby also tends to be larger due to sedentary lifestyle during pregnancy, higher social class and increasing parity (i.e. 2nd babies are larger than 1st ones, 3rd babies larger than the 2nd and so on).