You don't think it would have been better if he had been carrying 14-17 10-round magazines? You can only carry so many magazines at once, no? 17 magazines is a lot to carry on your person, and if it took 500 rounds to kill 50 people, wouldn't it be better if we were able to limit the amount of rounds a person could easily carry on their person at once?
Why can you only carry x number of magazines. This isn't a video game where there is some rule. The weight difference is negligible because the bullets weigh more than the mags, and the smaller mags would weigh less. Size wise, they're smaller too. Think about it. A 10 round magazine will be roughly 1/3 the size of a 30 round magazine, so it is roughly 1/3 the volume and length (thickness and width would stay the same).