Esters with high molecular weight have weaker fragrances because as the molecular weight of an ester increases, its vapor pressure decreases.

Vapor pressure is a measure of the tendency of a substance to evaporate. The higher the vapor pressure, the more volatile a substance is. Volatile substances evaporate more easily and have stronger fragrances. Esters with low molecular weight are more volatile and have stronger fragrances because they evaporate more easily.

Additionally, the solubility of esters in water decreases with increasing molecular weight. Esters with high molecular weights are less soluble in water and therefore less able to disperse in the air, resulting in a weaker fragrance.