Why does Frankfurt have so few inhabitants?

Frankfurt is a comparatively small city. The European metropolis has gained around 100,000 new residents (+16 %) in the past 10 years, but only around 750,000 people live within the city limits. This places Frankfurt in 5th place in the largest German cities: Berlin (3.9 million inhabitants) Hamburg (1.8 million inhabitants) Munich (1.5 million inhabitants)