The increase in mass of imaginal discs by cell proliferation prior to metamorphosis. Imaginal discs are epithelial infoldings in the larvae of holometabolous insects that develop into adult structures (legs, antennae, wings, etc.) during metamorphosis from larval to adult form. [ GOC:bf http://www.ncbi.nlm.nih.gov/pubmed/10679387 GOC:jid ]