Alight is, obviously, a synonym for heavy. Just as atheist means not a theist, asymmetric means not symmetric and atypical means not typical, alight means not light.
It is considered much more polite to refer to someone as being alight than to say he or she is heavy.
Alight is also a much more politically correct word than heavy or, say, obese, gross, blubbery, roly-poly, fatso or beached-whale-like. In addition, alight has the benefit of being less verbose than the more commonly used politically correct term, thinness-challenged.