IPA: /ˈʌndərˌsaɪzd/
KK: /ˈʌndərˌsaɪzd/
Having a size that is smaller than what is usual or expected.
The undersized shirt did not fit him well.
Undersized → It is formed from "under-" (meaning below or less than) and "sized" (from the root "size", which comes from Old French *size*, meaning measurement or dimension). The word "undersized" means having a size that is smaller than what is considered normal or appropriate.
Think of something that is 'below' ('under-') the normal 'measurement' ('size') — that's why undersized means smaller than usual.