Why is a hydrometer for lighter liquids not used for measuring densities of heavier liquids?

Hydrometers works on the basis of law of floatation.

The hydrometer for lighter liquids has a bigger floatation bulb and a smaller gravity bulb as compared with those hydrometers for lighter liquids.

The larger floatation bulb of hydrometers for lighter liquids helps to displace large volume of a given liquid. Hence, hydrometer on the whole does not sink in lighter liquids. The small gravity bulb ensures that the metacentre will be above the centre of gravity. Hence, a hydrometer for lighter liquids is not used for measuring densities of heavier liquids.