According to Oxford Reference, this was a "proverbial saying" in the early 19th century. Oxford's earliest attribution is to the Comte de Bussy–Rabutin (1618–93 – a member of the French lesser nobility, and a notorious rake): "God is usually on the side of the big squadrons against the small."
The exact quotation from Voltaire that I include on my main page is from a letter that he wrote to a wellwisher in 1770 (when he was 76 years of age and exiled in Switzerland, his anti–Christian views having made it impossible for him to live in France). According to Wikiquote, Voltaire wrote in his notebooks (between about 1735 and about 1750) that "God is not on the side of the big battalions, but on the side of those who shoot best."
© Haydn Thompson 2020