I am not surprised that the Mormon pages come out as letter size as that is the US standard and the Mormons are American.
You may be best letting the pages stay as they are because not only is letter paper a different size it is a also different shape and any tables or other things requiring a precise layout may be spoiled by forcing an alteration in page size/shape.
Having said all that, though, is it so important?
US small letter is 21.59 x 27.94 whilst A4 is 21 x 29.7. If there is a fit problem try altering the margins.