I have an SSD from Samsung, the 830 128GB.
It has windows and BF3 on it.
I check, went one by one and it stacks up to about 57 GB of real used memory, while it shows on Properties that 74 is actually used.
Don't confuse "memory" with storage. Memory is RAM. HDDs & SSDs are storage devices. Formatting a storage device occupies about 10% of the available space.