Andy Jassy spent years running one of the fastest-growing divisions in a company famed for its rapid expansion. Nearly a year after taking over as chief executive of Amazon he’s learning how to tap the brakes. Amid one of the worst stretches for financial performance in Amazon’s history, Mr. Jassy is working to cut back the excesses of an e-commerce operation the company expanded at breakneck pace during much of the Covid-19 pandemic. At the same time, he’s trying to resuscitate languishing sales in that business and drive growth in other divisions.