Sorry what? 'Light' and 'heavy' can be used as relative terms, just like big and small, hot and cold, or pretty much any adjective in English. If you say 'the lightest substance', it's quite clear that you are talking in relative terms. Also I think you are confusing 'substance' and 'object' - a single neutrino is not a substance, it is an object. Water is a substance; a drop of water is an object.
I mentioned gravity to deflect any possible nitpick about 'heavy' being different depending on what gravity you measure it in - since we normally understand heaviness as the force of an object being pulled towards the ground, in other words as a measure of weight rather than mass.