The problem is that you are confusing binary and decimal prefixes.
You said it was 750GB right? That means 750,000,000,000B. Now convert that to GB:
750000000000/1024/1024/1024 = 698.5GB
That’s what you said you were seeing. You can repeat this for the individual partitions and you will get the same results.
Why does Linux give you different results? Because it is not dividing by 1,024 (the number of bytes in a kilobyte, number of kilobytes in a megabyte, etc.), but rather, dividing by 1,000.
This is a frequent point of confusion because hard-drive manufacturers always use SI prefixes (which go up in increments of 1,000, i.e., powers of 10) because it makes their drives sound larger, while software (especially Windows) uses binary prefixes (which go up in increments of 1,024, i.e., powers of 2).
So, you are not actually missing anything, nor is there anything to "get back". The "missing" space is simply due to a difference/disagreement in the interpretation of the term "gigabyte/GB".
As to why you saw it say 750GB before installing Windows 8, it depends on where you saw that. Did you see it say 750GB in My Computer? Linux? A system-information tool? Based on what you have said, I suspect it to be a Windows program as opposed to Windows explorer.
In this case, you may saw it say 750GB because the program you used is clever enough to be able to ignore the disk-space reported by Windows and instead look at the device id of the drive. Then it can look up the id in a table (or decode it, depending on the device), and use the official label and description for it.
As an example, let’s say that you have a Western Digital 750GB drive. The model number is encoded in the drive’s firmware, so Windows is able to see something like
WD7500BPKT. Windows also sees that it has 750GB (decimal) of space available and stores that in binary as 698.5GB (or GiB if you like the baby-talk binary prefixes).
Simple programs like Explorer retrieve the free-space value and report that (698.5GB in this case).
A fancier program on the other hand does not ask Windows for the free-space value, but rather the device-id. It sees
WD7500BPKT and either decodes it (in the case of WD drives, they encode the size in the model number), or else it looks it up in its database and says; hey, this is a
Western Digital WD Scorpio Black 750 GB SATA 3 GB. Note the 750GB.
Alternately, a program may also just examine the total number of bytes and divide (repeatedly) by 1,000 instead of 1,024 to report the size as stated by the manufacturer. This is common with Windows ports of Unix tools.