Wait, the video review says in the 32GB model, there are 25GB of space left (OS takes up ~7GB), but in the 64GB there's 50GB left??? How does that make sense....just cuz the storage doubles, doesn't mean the OS takes up twice as much space as well. Hoping there's 57GB left in the 64GB model and it was just a misspeak
Have you ever defragmented a hard drive? Ever notice how if your drive is completely full, it won't run?
The idea is the same, and it applies here. The system needs to reserve a certain percentage of space to maintain operations. There is always an overhead.... Bigger the driver, bigger the overhead requirement.
Actually, it has a lot more to do with the actual physical capacity than "overhead". Let me explain.
The capacity of a storage device is expressed using the metric system. So, 1 Gigabyte = 1 billion bytes. However, computers use binary system to show storage capacity, and that's where you lose some "bytes".
In metric, 1 kilo = 1000 (10^3)
However, in binary, 1 kilo = 1024 (2^10)
So, in computer terms, 1 KB = 1024 bytes, while in metric, 1 KB = 1000 bytes, see how it takes 24 extra bytes for every KB?
Now, 32 GB storage has the physical capacity to store 32 billion bytes approximately (due to manufacturing it's never exactly 32 billion, but typically always a little more and not less)
But in computer terms,
32000000000 bytes = 31250000 KB (32000000000 ÷ 1024) = 30517.58 MB (31250000 ÷ 1024) = 29.80 GB (30517.58 ÷ 1024).
As you can see, a storage device with a metric capacity of 32GB can actually store 29.80 GB of binary data. For 64 GB, the actual binary capacity is 59.60 GB
If you take the info from the unboxing video (screenshot of the storage settings), it looks like the OS is about 4.79 GB (29.80 - 25.01). From that, you can expect about 59.60 - 4.79 = 54.81 GB available in the 64GB versions.
The guy in the video simply is clueless about what I just explained. He thought, if 32 GB has 25 GB available, 64 must have 50.. Lol..
BTW, because of the confusion between the commercial (metric) and binary capacity, there is another term GiB (MiB, KiB) that is used to show the actual binary capacity. So, 32 GB = 29.80 GiB. Look it up.
Hope it cleared up the confusion.
Edit: For comparison, Nexus 6 32GB has 25.98 GiB and 64 GB has 54.65 GiB available. Because of the variations in manufacturing, the actual number of bytes (as opposed to stated capacity) vary among different manufacturers as well as chip sizes.