I think that there may be other factors affecting the perception of size. The Marsh Sandpiper has a longer bill and neck, which will be included in the measurements for length, so the body size may not be so different. This Greater Sandplover also seems relatively fat, perhaps because it is preparing for migration.
Also individual birds will vary slightly in size - the measurements in a field guide are only an indication, not all birds will be exactly the same as each other (remember that not all people are the same height!). Some field guides give a range of sizes.
The measurements you quote are actually fairly similar, and indicate that these two species are similar in size. I think that this can be seen from your photo.
