mə-: "a small…" — the prefix that shrinks things.
The prefix mə- does not only mark the singular: it also carries a diminutive value. mətɯ́tɯ́ is not just "a tree" — it is "a small tree".
In Pawou Molu's analysis (2018), this diminutive occupies a very specific slot at the left edge of the noun, where the gender marker would be expected. When gender is not expressed, the diminutive takes that place — hence the gender / number / diminutive syncretism observed in Bamum.
🎓 This is the core of the thesis from which this module draws its grammatical description: Bamum merges into a single prefix information that other languages keep apart.