The English translation is odd to me. I've never heard lightweight used this way. Usually it's used with "a" as in "he's a lightweight." Or one might refer to fabric as a lightweight fabric. But if you want to say someone or something is the opposite of heavy, you say, "He/she/it is light." Is this a difference in usage between the US (where I am) and Great Britain?
The word Katze is grammatically feminine in German.
There is a word Kater which is masculine which specifically refers to a tomcat (i.e. a male cat), but if the sex of the cat is not relevant, I would use Katze even it happens to be a male cat. (Much as I would not use the word "tomcat" much in English.)