Pretty good article
here if you get bored and feel like brushing up on graphics card hardware.
In short, don't worry about it. It's most certainly calculating the total size based on RAM and maybe even page size. You won't be able to control it as it's a fundamental tenant of the architecture. Graphics card RAM is typically used to store items that are required regularly, at a high speed.
While Dead Eyes point is correct, it won't have any bearing on this situation. The graphics for a card is dedicated to the card itself, not seated externally.
If you still want more information, run a benchmark tool and it'll put your card through it's paces and provide a detailed report (such as how much RAM can actually be accessed).