The more they grow, the less YC gets for the 375k. But the more they grow, the higher the value of 7% is going to be. And also, the more they grow, the more they are likely to grow in the future. So the 375k share is also more likely to keep growing.
So in a nutshell: the 375k is incentive for the company to grow, which is also in the interests of YC, since they have 7% (+ x%) and in general getting startups to grow is the whole point of YC.
The company gets the money now.
The more they grow, the less YC gets for the 375k. But the more they grow, the higher the value of 7% is going to be. And also, the more they grow, the more they are likely to grow in the future. So the 375k share is also more likely to keep growing.
So in a nutshell: the 375k is incentive for the company to grow, which is also in the interests of YC, since they have 7% (+ x%) and in general getting startups to grow is the whole point of YC.