Don’t assume growing is automatically a good thing. Once you get beyond a dozen or so clients, you’ll have to bring on other staff—freelancers and then full-time. Freelancers are usually horribly flaky (which is why they freelance), so you’ll be spending most of your time managing them, instead of doing what you love.
Do you love management? Better get used to it—managing payroll, contracts, accounts receivable, project schedules, hire/fire, and the operations of running a business that has more than one person. Get bigger and you have to deal with a physical office.
Big clients can be big headaches. With a small business client, you have one person to deal with—the business owner. But with a major brand, you have to work through multiple levels of decision-makers, a contract process that may take 3 months, and a payment process that can take even longer. It’s sexy to say you have a global brand as a client—make sure you have the resources and stomach to handle it.
As a small agency or freelancer, you can develop closer relationships with clients. You don’t risk having so many clients that you’re working 24-7 to keep up.