Why is high molar molar mass a requirement of good primary standards for volumetric analysis?
This practice is done in analytical chemistry in order to minimize the relative weighing error on the balance. Preferring a larger formula weight for a primary standard has nothing to do with impurity levels. We have to start with the highest purity standard. 

For example, you wish to prepare a 0.010 M solution of oxalic acid dihydrate in 1 L flask. Its formula weight is  ‎126.07 g/mol. You need to weigh out 1.2607 grams of the acid on the balance. Imagine the balance has a small constant error, the relative error in weighing is smaller for a larger formula weight compound as compared to a smaller formula weight standard for the same concentration. Imagine if the molecular weight of oxalic acid were 60 g/mol, how much do you need to weigh out?