Why is it “ich” and not “mich” in the sentence “Er war kleiner als ich”?

Question

Every time I think I understand accusative, I find something that pokes holes in my understanding.

Most recently, I’ve operated on the understanding that any time you have two distinct entities (e.g., in Ich habe einen Bruder) one of them must be accusative (in this case, Bruder which we see in the -en addition to the indefinite article).

This does not apply to Er war kleiner als ich, though, and I’m not sure why?

I wrote the “incorrect” version in an edit (Er war kleiner als mich), and someone very reputable corrected it as ich.

Answer 1

The idea that anytime there are two distinct entities, one of them must be accusative, is wrong.

In general, it's the verb that determines the cases ("the verb governs the sentence"). For example, "haben" requires an accusative object. That's why you never learn single verbs, but always complete expressions ("etwas/jemanden (Akk) haben").

Occasionally, there are even verbs that require two accusative objects ("jemanden (Akk) etwas (Akk) nennen"). And for the verb "sein", both the subject and the Prädikatsnomen are in the nominative ("Ich (Nom) bin sein Bruder (Nom)").

Now for a comparison using "als", the things or persons that are compared are always in the same case. Which cases to use depends again on the verb:

Er (Nom) war kleiner als ich (Nom)

Ich gebe lieber ihm (Dat) als ihr (Dat) das Geschenk.

Ich liebe ihn (Akk) mehr als sie (Akk).

Answer 2

The role of kleiner als is comparison. It only works correctly when both cases are equal. You sort of write a bracket around the packet

[1st. noun + als + kleiner + 2nd. noun]

and instert it in the sentence, according to the case it needs.

But they don't need to be both nominative, as in

[Er war kleiner als ich].

They might be also both accusative (if applies)

Sie fand [ihn kleiner als mich].

In both example sentences the first noun is er, and the second ich. And the pieces in brackets are of the form described above.