MRGBZGrabber by default auto-crops to the pixols on the canvas. So if your object occupies only part of the canvas the resulting grab will be smaller than the canvas size. To change this behavior, turn off Tool>Modifiers>Auto Crop. In that case it will then capture the entire area within your grab area regardless of whether it’s empty or not.
Either way, the captured alpha is always going to be 16-bit grayscale. The level of detail is exactly the same either way.
Another way to capture the canvas is using Alpha>GrabDoc. This will always capture the entire canvas.