TORONTO/PARIS—Bank of Montreal said on Monday it will buy BNP Paribas’ U.S. unit, Bank of the West, for $16.3 billion in a deal that will allow the Canadian bank to double its footprint in the world’s biggest economy, while giving France’s biggest bank a huge step up in firepower for deals. The deal gives BMO, Canada’s fourth-largest lender, a large-scale presence in California, whose population is bigger than the bank’s home country. It will also increase the United States’ contribution to BMO’s pre-provision, pretax earnings to 44 percent from 36 percent in fiscal 2021 and will give the Canadian bank the ability to deploy almost all of its excess capital, which has been a drag on returns. BNP Paribas has been struggling to keep up with larger rivals in the U.S. retail banking market, and the sale will leave the French bank focused squarely on Europe, where it is growing …