What eawmp1 is saying, regardless of your iPhone size, whether it be 16 GB or 32GB, that number is a set number. You will always have 16 GB, or 32GB, or whatever, depending on the iPhone you choose to buy.
Now, whatever you decide to put into your iPhone is a different story. You could choose to have 16 GB of pure music, or 8 GB movies and 8 GB movies, or any random assortment of combinations, just like a purse. you can fill your purse with whatever you want, depending on the size of your purse, but what you want and need is what you will end up putting into your purse.
It's not an argument of what the weight of the iPhone itself is. it's an argument of whether or not the additional 16GB of space is a necessity, depending on what you need to put into your iPhone.
While, yes, the 32 GB iPhone can store more things and always will be a benefit because of that, the price difference vs. usage of that additional space is a different story.