Why are enzymes large molecules?

Answer 1

Because they're proteins. Proteins = made from many amino acids

Answer 2

Enzymes are proteins, which are made from several amino acid units. More specifically, enzymes are proteins that catalyze chemical reactions. They do so (usually) by forcing the reacting molecules into a 'transition stage analog'. To do this, the enzyme has to wrap around the chemicals that are reacting, so it has to be much larger than they are.