The 1906 Jewish Encyclopedia [answers][1] that the division of the several treatises into chapters as well as the sequence of these chapters was the work of Rabbi [Yehuda HaNasi] himself although some changes were made over time. See [there][1] for full references.

> **The division of the several treatises into chapters as well as the
> sequence of these chapters was the work of Rabbi himself** (Letter of
> Sherira Gaon, l.c. p. 13). The portion discussed each day constituted
> an independent pereḳ; and this term was, therefore, applied elsewhere
> to a single discourse also. **Generally speaking, the original division
> and sequence of the chapters have been preserved**, as appears from
> various passages of the Talmud (R. H. 31b; Suk. 22b; Yeb. 9a; Ket.
> 15a; Niddah 68b; Zeb. 15a).
> 
> The names of the chapters taken from the initial letters are likewise
> old, and some of them are mentioned even in the Talmud (B. M. 35b;
> Niddah 48a). **In the course of time, however, various changes were made
> in the division, sequence, and names of the chapters**; thus, for
> example, the division of Tamid into seven chapters is not the original
> one. On other variations in sequence see Frankel, l.c. pp. 264-265,
> and on the changes in the names see Berliner in "Ha-Misderonah," i.
> 40b.

See also the Rambam's [introduction to the Mishna 15:15][2] where he writes

> Afterward, [R Yehuda Hanasi] divided the material that is in each of the specific
> categories into sections and he called each one, chapter. And
> afterward, he divided the material of each chapter into small sections
> that are clear to understand and easy to know by heart and to teach,
> and he called the name of each one of these small sections, law
> (halacha [more commonly referred to as mishnah]).

I remain [puzzled][3] about the logic for cutting chapters this way and don't yet understand the rules used.


  [1]: http://www.jewishencyclopedia.com/articles/10879-mishnah#anchor14
  [2]: https://www.sefaria.org/Rambam_Introduction_to_the_Mishnah.15.15?lang=bi&with=all&lang2=en
  [3]: https://judaism.stackexchange.com/q/93594/11501