IPA: /ˈʌndərˌsaɪz/
KK: /ˈʌndərˌsaɪz/
Smaller than what is normal or expected.
The undersize shirt did not fit him well.
Undersize → It is formed from "under-" (meaning below or beneath) and "size" (from Old French *size*, meaning measurement or dimension). The word "undersize" means below the normal size or smaller than expected.
Think of something that is 'below' ('under-') the normal 'measurement' ('size') — that's why undersize means smaller than usual.