Iba a poner esto en los comentarios, pero salía demasiado largo. Lo pongo pues como respuesta.
En Linguist ante la pregunta:
Has there been any research done on the subject of cross-linguistic
variation sentence-length? I have been specifically comparing Finnish,
German, and English.
More specifically, I am interested in cross-linguistic comparisons of
sentence length in formal writing.
Responden:
(...) There are, however, systematic differences between languages, even
between such relatively close relatives as English and Spanish. For
example, Spanish tends to often have a comma joining two 'sentences'
(clauses) where English would have to have a period. This is standard
and acceptable for Spanish. This and other differences mean that
sentence length in words tends to be longer in Spanish than in
English, so, for example, one has to 'tweak' the readability formulas
for English to reflect such differences encountered, in this case, in
Spanish.
Lo que sigue también es interesante, pero no lo copio aquí para no hacerlo demasiado largo.
Sin embargo, creo interesante mencionar una parte posterior que tiene parte de "meta" en cuanto a cómo está planteada la pregunta en sí:
In short, you will have to define very carefully what you mean by
'length' (orthographic or according to a phonetic transcription [the
latter much harder to count in any sizable corpus, although I think an
automatic orthography translator to a phonetic representation should
be fairly trivial for Finnish]); these are not the same for most
languages.
También es interesante ver la pregunta Do most languages need more space than English? de English Language & Usage. Hay muchas respuestas interesantes, por lo que sugiero leerlas con calma. Me quedo con algún dato:
Spanish, Portuguese and French (I guess we can just settle on Romance)
texts are longer than their English counterparts by about 1/5 to 1/4.
Sin embargo, esto tiene más que ver con la longitud de las palabras que con la de las frases.