Doesn't matter how many you pay for and actually get. What matters, is how many you have access to. The larger the drive you have, the higher the net loss in terms of how much space you don't have access to.
That use to be true (in a way) under old file systems like FAT, where when the size of the drive went up, the cluster size had to go up as well.
That is no longer the case with NTFS - cluster size is fixed at 4K by default.
As far as accessing the space, you have access to all of it -- no problem there. One thing that can vary is the size of the NTFS MFT. The MFT is initially allocated 12% of the volume (although I don't think Windows subtracts this from the reported free space).
However, if the volume fills up, Windows will release reserved space from the MFT to use for file storage, just like if the MFT runs out of space, Windows will allocate additional space for it.
NTFS reserves the space for the MFT at format time, because Windows wants to take extra steps to keep the MFT defragmented. A fragmented MFT can really hurt performance, and all Windows defragmenters cannot defragment the MFT - to defrag it, you need a DOS or boot-time utility.
But, if you purchase a 320GB drive, you will be able to store a single file very close to 320,000,000,000 bytes on it, just like if you buy a 750GB drive, you will be able to store a file very close to 750,000,000,000 bytes on it. Yes, Windows will report free space on those disks after a fresh format as 298GB and 698GB, respectively, but that is the result of 1000/1024 computational differences. It doesn't affect the actual amount of space (in bytes) available on the volume.