The segmentation’s resolution matches the master volume’s resolution but the extent is cropped to the minimum necessary by default (the origin is adjusted accordingly, so the segment’s physical position remains correct). You can export the segmentation to labelmap volume before saving it if you prefer saving the blank parts of the segmentation (so that extents are exactly the same as the master volume’s).
We see that many users are surprised by this behavior, so we will change it to not crop to minimum extent by default (see Segmentation image size).