Maximum memory usage in Final Cut Pro is 2.5 GB
On a computer with more than 4 GB of RAM installed, you may notice that Final Cut Pro allows a maximum memory allocation of 2.5 GB.
This is what you'll see in the Memory & Cache tab of the System Settings window:
Why can't Final Cut Pro use more than 4 GB of RAM?
Mac OS X is, at present, a 32-bit addressable operating system. This means that any single program running on the computer can take at most 2^32 bytes, or 4 GB, of memory. Final Cut Pro is subject to this ceiling just as any other program would be and is restricted to allocating no more than a total of 4 GB of RAM.
Why can't I allocate more than 2.5 GB of RAM to Final Cut Pro?
The memory available for allocating in the Memory & Cache tab of the System Settings window includes memory directly used by Final Cut Pro, but the program loads many other frameworks, libraries, and drivers, which may also allocate memory. Although this memory isn't directly allocated by the program itself, the allocation still falls into Final Cut Pro's address space and contributes to the 4 GB limit. Final Cut Pro reserves 1.5 GB of RAM for these frameworks, libraries, and drivers and so only allows a maximum of 2.5 GB to be allocated by the user.