Fortune is always on the side of the largest battalions.

[La fortune est toujours pour les gros bataillons]

Marie de Rabutin-Chantal, Marquise de Sévigné (1626-1696) French aristocrat, woman of letters [Madame de Sévigné, Mme de Sévigné]

Letter (1673-12-22) to Mme. de Grignan [ed. Hoyt and Ward (1896), No. 202]
    (Source)

(Source (French), No. 118). Other translations:

Fortune generally declares in favor of numerous battalions.
[Source (1811), No. 274]

Providence is always on the side of the big battalions.
[Oxford Dict. of Proverbs]

Also attributed to her cousin Roger de Bussy-Rabutin ("God is usually on the side of the big squadrons against the small").

The phrase had become proverbial by at least the early 19th C. Other variants include:

• God sides with the big battalions.
• God sides with whichever side has the biggest battalions.