When the cells of a garlic clove are broken open by slicing, crushing, or chewing, an enzyme (alliinase) in the cell vacuoles spills out and reacts with a precursor compound (alliin) in another part of the cell to form diallylthiosulfinate (allicin) and other thiosulfinates, which are the main odoriferous and flavor compounds.
Until relatively recently-1993, it was thought that the odorous compounds were diallyl disulfide and other polysulfides, but it has been shown that these were merely the breakdown products of thiosulfinates, and were inadvertently created in the analytical laboratory by the high-temperature methods being used. (A scientific research principle: Always make sure that your analysis procedure isn’t changing what you’re trying to analyze.)
Thus, when garlic is cut into smaller and smaller pieces, or reduced even further by being crushed, more vacuoles are broken open, more alliinase enzyme is released, and more thiosulfinates are formed, producing a stronger aroma and flavor.
A similar sequence of events takes place in sliced and chopped onions, initiated by the same enzyme, alliinase, but producing somewhat different flavor and odor compounds.
So when a recipe specifies a paste from a garlic press, or slivers of cloves, or chopped garlic, pay attention; otherwise you may get more or less than you bargained for.