Heavy cream contains a higher percentage of milk fat (usually called butterfat, because butter can be made from it) than light cream does: 36 to 40 percent fat in heavy whipping cream versus only 18 to 30 percent in light cream.
And, if you’re interested, the heavy cream can contain up to twice as much cholesterol. But volume for volume, fats weigh less than water; they’re less dense. So the higher the percentage of fat in a water-based liquid, the lighter the whole liquid will be.
It’s not a huge difference. In my kitchen laboratory, a pint of heavy whipping cream weighed 475.0 grams, while a pint of light cream weighed 476.4 grams: three-tenths of a percent heavier.
The names “heavy” and “light” as related to cream were never meant to signify weight; they apply to richness or thickness.
Fattier substances are thicker, more viscous, and therefore feel more substantial or “heavier” on the tongue.