File size and resolution must be the same. Then the layer size will be preserved. Only Photoshop CS2 allowed not to change the size of the smart object when replacing content. Then some smart guy didn't like it and they decided to redo it as it works now. You need to transform the smart object after the Relink command to the dimensions that were before this operation. Everything would be fine, but sometimes the size of the layer after Relink (Replace) can turn out to be so gigantic that you may simply not have enough memory to perform the transformation, and a lot of time will be spent on it.