The D3000 has an FX sensor. But if you place a DX type lens on the camera, the camera will only process the DX sensor sized area of the FX sensor and produce an image that has all the attributes of the DX sensor image. So your image has the DX crop, smaller megapixel count of the image, and a smaller file size due to the smaller megapixel size of the image.
Technically this could be considered as a 'mode' of the camera.
You can override this camera behavior and force the camera into the FX mode while you have a DX lens mounted on the camera. Depending upon the lens mounted and zoomed focal length, if a zoom lens, you will either have a circular image or a normal image depending upon the image projected by the lens.