The first distinction is evident in the name; the grand form was truly grand (big) in proportion calling for double choirs and massed orchestral forces, whereas the petit form was a chamber genre for one or two solo voices, one or two solo instruments, and basso continuo - the basso continuo typically provided by the harpsichord at home or organ in a smaller church.
These grand motets extend and elaborate upon preexisting conventions; they epitomize Versailles style and are lengthier and more musically interesting than their forerunners.
The motet Versaillais, which reached its apex under the talent of Michel Richard Delalande, is characterized by its unprecedented length (Lully's Te Deum, for instance, has over 1200 bars) and a sectional structure that incorporates the alternation of a grand choeur with a petit choeur composed of (at least) four soloists.
The first generation of grands motets, the works of Henry Du Mont, were purely royal occasions.
Virtually all major, and some minor, composers of the French baroque tried their hand at the genre, but only performance at court or, later, the Concert spirituel conferred approval.