Anecdotally, when I worked at a 'startup' and we were about 50 people, the CEO and other founders were always very open and forthcoming about all the details of the organisation.
Once we grew to over a 100, IMHO, a combination of factors inhibited their ability to be that open:
1. we moved to a larger office with multiple stories, which put a little extra cost into simply walking across and asking questions.
2. it was a different sort of challenge communicating a common message across to that many people and they couldn't figure out how to 'scale' the openness.
Eventually I noticed that rumours about salaries and customer nonpayments and layoffs and revenues and profits started up. It was still entirely possible to simply ask the founders about a rumour, but not everyone believed/knew this was possible (especially newer hires) and so untruths would circulate for longer than was healthy.
I never got over the 25 employees mark and probably that is one of the reasons why this problem never occurred but I can see your point and I think that once you reach a certain threshold (50 employees or so?) that it could be very hard to maintain such a culture. I'd also argue at the same time that once you do reach that level the moniker 'start-up' no longer applies to you. By that time you should have a solid business, the biggest risks should be behind you and you're transforming into a 'normal' company.
This line of thinking leads to yet another way to delineate what a start-up is: a start-up is a company where everybody knows everybody else.
For example Ryan Air is quite open (at least compared to other airlines) and it´s very big. With some thousands of employees, they send monthly letters informing on how many people is joining, how many will go, if a base is closing due to poor performance, etc..
In traditional airlines, all this info is purely gossip, and you never know what is really happening.
Once we grew to over a 100, IMHO, a combination of factors inhibited their ability to be that open:
1. we moved to a larger office with multiple stories, which put a little extra cost into simply walking across and asking questions.
2. it was a different sort of challenge communicating a common message across to that many people and they couldn't figure out how to 'scale' the openness.
Eventually I noticed that rumours about salaries and customer nonpayments and layoffs and revenues and profits started up. It was still entirely possible to simply ask the founders about a rumour, but not everyone believed/knew this was possible (especially newer hires) and so untruths would circulate for longer than was healthy.