WordNet records no synonym and no antonym for downsize. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
dismiss from work
“three secretaries were downsized during the financial crisis”
design or manufacture in a smaller size
“the car makers downsized the SUVs when fuel became very expensive”
(of a company) reduce in size or number of employees
“the company downsized its research staff”
Synonyms, antonyms and the broader and narrower relations for downsize come from Princeton University’s WordNet. Full sources and licences.