HELSINKI—Petteri Koponen visited a dingy office on the outskirts of Finland's capital in 2010 to meet with a group of developers launching a mobile-games company called Supercell.

"Basically it was just one room with very small windows," recalls Mr. Koponen, an early investor in startups. "At that point, they didn't have chairs for everyone." The company's co-founder, Ilkka Paananen, was hunched over a cardboard box he used as a desk as he discussed the company's plans.

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