Its only AFTER the end of WWII, when Oskay desides to grow again, just as he decided at age 3 to STOP, that you get the ugly, deformed dwarf.

In the first 100 pages, I kept wondering why Mr. Grass had chosen to write the novel in the form of an autobiography of an insane dwarf pretending to have a mental age of 3 who had been convicted of a murder he did not commit.

Why is it that people consider OSKAR the insane dwarf

He claims to be a self-imposed dwarf that can choose to grow again, and also claims to do so midway into the story, only to halt his vertical progress once more by his sheer will alone.

The Onion Cellar is as flippant an interpretation of "German guilt" as I've ever read; yet it rings true, for that time as well as now.