The process of conversion of higher molecular weight hydrocarbon into lower molecular weight hydrocarbon is called

Cracking is the petroleum refining process that breaks larger, higher molecular weight hydrocarbon molecules into smaller, lower molecular weight molecules. This is done through thermal cracking (heat) or catalytic cracking (catalysts). Isomerisation rearranges molecular structure, aromatisation creates aromatic rings, and cyclisation forms ring structures - none of these reduce molecular weight like cracking does.