Why does a texture2d-array preoccupy more space than specified?

Let’s say I create a Texture2DArray like so

var texArray = new Texture2D(GraphicsDevice, 4, 128 * 128, false, SurfaceFormat.Color, ARRAY_SIZE);

Now let’s assume ARRAY_SIZE is 1000, the overall size of the texture-object one would expect should be:

(4 * 128 * 128) * (1000 times) * (4 BYTE) = 0.262144 gigabytes.
But for some unknown reason it takes up to ~1 gigabytes… why?

Interesting: Changing it to var texArray = new Texture2D(GraphicsDevice, 128 * 128, 4, false, SurfaceFormat.Color, ARRAY_SIZE);

reduces it down to ~500 mb.