"The only Brandenburg Concerto in four movements, the First may appear to be the conventional fast-slow-fast form to which a final dance section was added, but scholars trace a more complex origin, in which the first, second and fourth movements comprised a "sinfonia" to introduce a 1713 Hunting Cantata and thus was more like a standard suite of the time. Boyd goes further to speculate that to create the character of a concerto, Bach later added the present third movement with its prominent violin solo, the short phrasing of which suggests separate origin as a now-lost choral piece. (Bach later adapted the third movement back to a choral setting to open a 1726 cantata.)
The overall orchestration is unusual. The sheer number of instruments gives the work more of an orchestral than chamber character. Karl Geiringer calls it a "concerto symphony." To expand the range of the sonority, Bach specifies in lieu of his standard violone a "violone grosso" played an octave below the bass staff (corresponding to the modern double bass) and in lieu of a solo violin a "violono piccolo," an obsolete small violin with scordaturo tuning a major third above notation and whose lighter bow, less resonant body and tighter string tension yield a sweeter, lighter tone. Harnoncourt asserts that the instrument was chosen purely for its tone color, rather than any technical reason. Indeed, Boyd notes that Bach didn't exploit its higher range and that its reduced volume is overwhelmed by the large ensemble. Bach himself used a regular violin in his earlier sinfonia version."