This sounds more like a computer issue. I have 8GB of RAM and in my dxdiag it shows Approx. Total Memory 4042 MB and dxdiag has nothing to do with ArmA2. ArmA2 don't change your "Approx. Total Memory". This is some kind of hardware/computer issue.
The Approx. Total Memory means that is the total memory from Dedicated Video Memory + Shared System Memory = Approx. Total Memory
Running the Microsoft DirectX Diagnostic Tool may show the incorrect total memory under the Display tab for the video card, as shown in the below picture. In this example, a NVIDIA GeForce GTX 670 with 2048MB of video memory is being reported as 4042 MB approx. total memory.
This is only the "approximate" total of memory the video card is using and will include not only the total video memory but also the shared system memory in the computer. Viewing the display properties for the video card will break down how the memory is used as shown in the below example. As can be seen in the below picture, the adapter properties show that the dedicated video memory as well as shared system memory.