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.