Which firms drive aggregate productivity growth? We document that firms with high price-earnings ratios tend to see increases in their subsequent earnings relative to sales, which we interpret as rents from ideas (innovation). We construct an endogenous growth model with shocks to firm innovation step-sizes and R&D efficiency and calibrate it to match patterns in the data. The model implies that growth would be much lower, even with the same innovative effort, if firms had the same step sizes. The model can be used to infer expected growth contributions of individual firms (such as members of the Magnificent Seven). We find that the share of growth coming from the smallest listed firms substantially exceeds their sales share, whereas the largest listed firms account for less than their sales share.