Java throwing out of memory exception before it's really out of memory?

I wish to make a large int array that very nearly fills all of the memory available to the JVM. Take this code, for instance:

    final int numBuffers = (int) ((runtime.freeMemory() - 200000L) / (BUFFER_SIZE));
    System.out.println(runtime.freeMemory());
    System.out.println(numBuffers*(BUFFER_SIZE/4)*4);
    buffers = new int[numBuffers*(BUFFER_SIZE / 4)];

When run with a heap size of 10M, this throws an OutOfMemoryException, despite the output from the printlns being:

9487176
9273344

I realise the array is going to have some overheads, but not 200k, surely? Why does java fail to allocate memory for something it claims to have enough space for? I have to set that constant that is subtracted to something around 4M before Java will run this (By which time the printlns are looking more like: 9487176 5472256 )

Even more bewilderingly, if I replace buffers with a 2D array:

buffers = new int[numBuffers][BUFFER_SIZE / 4];

Then it runs without complaint using the 200k subtraction shown above - even though the amount of integers being stored is the same in both arrays (And wouldn't the overheads on a 2D array be larger than that of a 1D array, since it's got all those references to other arrays to store).

Any ideas?