I'd guess that this phrase is metaphorical in some sense. Unless maybe it's a song about weight-loss. Supposing it is metaphorical, either option is possible. We often speak of the state of being burdened with problems as a sort of heaviness; the opposite state, that of being care-free can thus be described as lightness. At the same time, we sometimes think of happy people as having a cheery glow; so the person in the song may now be giving off some kind of warm light.