It's difficult ;) In Polish you have three gendrers (masculine, feminine and neuter) in singular and two genders in plural (masculine personal and non-masculine-personal).
Pomidor is masculine (ten pomidor) so in singular it's "duży pomidor". When you have some tomatoes, they become non-masculine-personal gender (te pomidory) and than you have "duże pomidory".
There are two plural categories in Polish, and pomidory belongs to nonvirile nouns (non-masculine-personal). The pronoun tamci would have been virile.