The logia fruits I have always taken to a natural force; they harness the nature of the "element" they are and bend it to their methods. Bronze is a creation of man from copper and tin, to have a bronze man you would need a tin man and a copper man who could use their powers together.
As for the ability to grow in size and being a logia, enel (as according to his one piece wiki page) grew in size by making his body into pure electricity. The only other case (correct me if I'm wrong) of a logia fully becoming their "element" is crocodile who turned into a sandstorm which was in terms an increase of his size. This may be coincidence but it also may mean the full transformation of a logia into their element will result in a dramatic size increase which would suggest Sengoku to be a logia but not of a bronze type.