As alluded to in comments, I suspect that the reduced error from scale may also play into it. Some of the ingredients added to the potions are fairly small amounts, and we see in the books that small differences sometimes seem to have large effects. If you need to add five drops of liquid to a gallon of potion material in the student recipe, scaling that down to, say, 8 oz, means that one accidental extra drop makes much more of a difference than one off from five.
As to the waste, wizards never seem to think too much about that. There are huge tables of food created out of thin air for each of the meals. Snape routinely disappears the contents of the cauldron, an effect noted to return the material into nothingness. Either something in the nature of magic means that there's some degree of recycling going on, at least to preserve a conservation of mass and energy, or the wizards just don't care, possibly because they don't understand the potential implications of their actions.