>
> Sorry, maybe it's too hot here to think (36°C currently), I still don't 
> get it
>
> (- (point-max) (point-min))  = current buffer size
>
> so
>
> current-buffer-size - last-buffer-size <= 1
>
> current-buffer-size <= 1 + last-buffer-size
>
> which is "the buffer got smaller"?
>

You are correct.  It's too hot here too, apparently!  😉