The density of the storage is higher. Put simply there are more NAND flash memory chips (or the chips can hold more). The chips are very small though, so there is some empty room in most USB sticks. Making the chip bigger, or adding a second is almost always possible in the same space.
To save space, NAND flash chips also often package two pieces of silicon to same “chip”, meaning the plastic/ceramic piece you usually call “chip” has one, two or four individual pieces of silicon inside. This comes with performance cost, but on USB stocks performance is not usually a concern. This all is made possible by transistors used to implement the memories shrinking all the time.
If you think about microSD cards, they are smaller than nail on my index finger, and they still come with capacities of 16 GB or more. Or SD cards, size of a stamp and 2 mm thin, come in capacities of 128 GB… And one SD card is easily smaller in physical volume than most USB sticks.
It is also a common practice in electronics design to first create the top-end model with all bells and whistles and then just not install some components to get the cheaper models; so these two sticks might very well be all identical, except one of them, while having space for another 8 GB die, does not come with it installed.
The reason for this is that designing the circuit board, testing the design, making molds for plastic parts, etc. is the expensive part. Making a single unit is cheap compared to that. Which means it makes sense to keep the plastic cover and circuit board identical, and just leave out some components, because you then save on the expensive part, materials.
The size of the USB stick is determined by the marketing people, the design department, and not necessarily the components inside it.
There are USB memory sticks that hold gigabytes of data that fit completely inside the USB port they are plugged into.
For a good example of just how small a thing large amounts of data can fit into, look at Micro-SD cards, which are used in many cell phones. The package is less than a centimeter on a side, only a mm or so thick, and can hold up to 32GB of data.
Generally, higher density components are more expensive. So a normal size USB memory stick doesn’t have to use the most expensive, most dense components.
USB sticks are generally designed to fit comfortably within the hand and pocket. Smaller devices are more prone to getting lost, and larger devices won’t fit in the pocket quite as well.
So in designing these devices, the company decides on a price point, which will govern to a certain extent how expensive the components can be, the design department decides on a branding and design and size, and then the technical people are told to make it work.
Inside the drive casing, there’s a circuit board with some logic chips and some memory chips on it. It’s likely that either:
- The 8 GB uses the same board but leaves off half the chips that the
16 GB has installed
- The 8 GB uses a different memory chip with half the
capacity, but a similar package (and inside that package, there’s half as much actual
circuitry, as soandos suggests)
A large fraction of the part cost for a USB drive is probably in the memory chip. All other things being equal, a 16 GB chip should cost slightly more than 2 8 GB chips (because compactness is worth something), so it’s not completely out of line to double the price when you double the capacity (especially at the high end of capacity, where the chip is a bigger part of the cost). Looking at one brand of USB drive with identical form factors, I’m seeing 4 GB for $15, 8 GB for $25, 16 GB for $40, 32 GB for $70 which is less than doubling at each step.
Manufacturers would save money if they can just use the same casing for all the size options and then just fill it with what is needed the out side of the case does not determine size of the storage. mainly because it is not practical to do it that way.
What gives the capacity of the USB drive is the amount of memory it has inside. The NAND memory is stored inside some chips. If you open your USB drive, you can see the chip inside it. Here’s a NAND chip:
Even chips of the same size can have a different amount of memory space inside it. So, just the physical size of the chip isn’t a indicative of it’s capacity.
As you can imagine, chips with more memory have to be built in a different way: more raw material, different layouts, different amounts of eletronic components. So, bigger ones will cost more.