This is excellent information Chromatix. Thank you for taking the time to do this.
I was surprised to notice such a big difference in the practical test results between the Sandisk blue 4G C4 card and the Sandisk blue 16G C4 card. Apart from capacity was there any other difference between these two cards?
Internally, I suspect that the 4GB card uses 2-bit MLC flash, perhaps from an older fab which is cheap to run, while the 16GB card uses 3-bit MLC - certainly the price I got it for strongly suggests that. This results in a considerably lower sequential throughput for the larger card, because 3-bit MLC takes *much* longer to erase than 2-bit MLC or (1-bit) SLC flash. You'll notice that it obtains Class 4 throughput with only a little to spare. It's a lot faster than that for reading.
The non-sequential performance of the larger card is still relatively good, suggesting that SanDisk have still used their advanced controller chip with it - a good way of attaining the other requirements of the Class 4 spec. This would also help a great deal with the longevity of the back-end flash, since the advanced controller can reduce write amplification by a lot compared to the more basic ones out there.
You see this a lot with SD cards. Notice that the 32GB Transcend Class 6 is similarly slower than the 4GB Transcend Class 6.
This is in direct contrast to other drives, which tend to increase in performance as they get bigger. With mechanical disks this is because there is more data per inch on each track, which is read at a constant *physical* speed, and the tracks are closer together too so the head has less far to move to each file on average. With SSDs the effect is smaller but is due to having more physical flash chips able to operate in parallel, so multiple erase and write operations can be going on at the same time.
The key to knowledge is not to rely on people to teach you it.