Whether it's grammatical or not, if it was used as a motto by a famous person or organization, you would expect to find it somewhere on the Internet or in Google Books. See this search for "subject:Mottoes" for a list of searchable books in this category.

I found nothing even close to "Parvum Numero Magnum Activo."

Penny S mentioned that she posted the motto "from memory." I hope she checks again and comes back with a report.