Even if they are cooks, it's not easy to parallelize every problem. If the money is mostly used to pay for massive infrastructure - e.g. google, fb, etc buying lots of servers and other equipment investment or king/supercell spending on user acquisition (=advertisement) you can spend practically as much money as you can raise into it.

But if your costs are mostly salaries putting a lot more people to work is a hard problem to solve.

I'm currently running a startup with 6 employees (myself included) and if i get some extra money i can easily grow to 8 and immediately find plenty for the new people to do. But if you give me $Xm and i can/should suddenly grow to 30 people I'm going to have a hard time putting them all to use. I may eventually get to it but I can't start hiring them today because they will just idle or do unimportant work until I figure that out.

Beyond that, even if you do have a good use of 5x as many people productivity grows in sublinearly to the number of employees due to communication overhead. So even a well run company (which the above example was not) will probably get less per-person out of 1000 people than out of 150.