void G1ParScanThreadState::report_promotion_event(InCSetState const dest_state,
                                                  oop const old, size_t word_sz, uint age,
                                                  HeapWord * const obj_ptr) const {
  PLAB* alloc_buf = _plab_allocator->alloc_buffer(dest_state);
  if (alloc_buf->contains(obj_ptr)) {
    _g1h->_gc_tracer_stw->report_promotion_in_new_plab_event(old->klass(), word_sz, age,
                                                             dest_state.value() == InCSetState::Old,
                                                             alloc_buf->word_sz());
  } else {
    _g1h->_gc_tracer_stw->report_promotion_outside_plab_event(old->klass(), word_sz, age,
                                                              dest_state.value() == InCSetState::Old);
  }
}

// word size == n

void YoungGCTracer::report_promotion_in_new_plab_event(Klass* klass, size_t obj_size, <<------------
                                                       uint age, bool tenured,
                                                       size_t plab_size) const {
  send_promotion_in_new_plab_event(klass, obj_size, age, tenured, plab_size);
}

Event describes an object with obj_size n, but n is still word_sz. Missing conversion / expansion.