I am always skeptical of relics;Gibbon reported enough claimed pieces of “The True Cross” to make a sizable wooden fleet. There has always been lots of relics to stimulate donations by poor souls.
Gibbon was being facetious or truly did not know what he was talking about. An inventory of relics done recently that took measurements of the relics of the "True Cross" found that there is sufficient surviving wood (if all are genuine, which is doubtful) to only reconstruct about two-thirds of the Patibulem (the horizontal cross piece). The Roman practice was to leave the Stripes (the upright posts) of their crosses permanently in place and only requiring the condemned person to carry the 100 pound cross piece to his or her execution site. The anti-relic remark by Gibbon was simply not based on fact, but there was a thriving market in fake relics for gullible pilgrims.