Why is the image buffer of my ImageReader callback so big?

Within my ImageReader.setOnImageAvailableListener() I have the following callback code:

previewImageReader.setOnImageAvailableListener({ reader ->
    val image = reader.acquireLatestImage()
    var buffer = img.planes[0].buffer
    buffer.rewind()
    var size = buffer.remaining()
    var bytes = ByteArray(size)
    buffer.get(bytes)
    // do stuff with the received ByteArray
}

Here is the weird thing: For testing purposes I created the surface of the image reader with extremely small dimensions:

previewImageReader = ImageReader.newInstance(4, 3, ImageFormat.JPEG, 1)

However, buffer.remaining() returns a number >70.000. Why is the buffer so big, even though the dimensions of the return image are 4x3 pixels?

from Why is the image buffer of my ImageReader callback so big?