This is absolutely a dead giveaway that we’re looking at the work of an artist. It’s easy to see how an artist could have painted the figures separately and accidentally made the back side of the figure longer than the front side, but it is hard to see how, if the shroud were authentic, Jesus’s back could have been longer than his front. That in itself would take some kind of miracle.
The size difference can be more accounted for as an artifact of how it was imaged. Jesus’ head was not flat, but was bent forward. So, it would cause more cloth to be exposed to the body from the back than on the front while the cloth was wrapped around the body.