In Gbaya, the notion of being heavy (or: strong) is emphasized in the referenced verses with ɗɔ́ŋ-ɗɔ́ŋ, an ideophone that denotes something that is heavy regardless of volume, i.e. it expresses volumeness, volumonosity.
Ideophones are a class of sound symbolic words expressing human sensation that are used as literary devices in many African languages. (Source: Philip Noss)