A deliberate exaggeration, which no one takes literally, to give emphasis to a feeling or a situation.
In Purgatory
for a month and a little more I tasted how heavy the great mantle is to one who guards it from the mud, so that all other burdens seem feathers.
In contrast with the mantle, "all other burdens" seem "feathers." The exaggeration gives the measure of the power that had once seemed to him the summit and that now makes everything else small.