When you select <Original> in the Export Dialog the export will be of the same size as the original. You said yourself that some photos are from your phone and some from your SLR. They create different file sizes. So your originals are of different file size. So your exports as <Original. are of different file size.
When you specify a size in the Export Dialog, all exported images will have this size.
If, for instance, you select 1000 pixels for the long side, all exported images will be 1000 pixels on the long side, but the short side may vary because its size is determined by the ratio of the sides. For example; if you had a square image, long side and short side would be 1000 pixels. But for an oblong picture the long side will be 1000 pixels but the short side may be 750 pixels.