It comes down to this: in a startup people usually act based on the social norms that generally only apply to close relationships; friends, relatives, that kind of thing. In a corporation, people act based on social norms that generally apply in semi-anonymous scenarios.
So when a person has to make a decision: do I take the choice that's best for me at the expense of the group, or the choice that's best for the group at my own expense; in a startup they will often side with the group, while in a corporation they will often focus on their own personal benefit. In a startup people will often accept lower pay and work harder; in a corporation people are more likely to call in sick when they're not and make decisions that yield less work for themselves even though it may be worse for everyone else.
There's a very interesting book called The Moral Foundation of Economic Behavior that talks about this phonomen, particularly in the context of startups, citing studies and figures and all that scientific stuff. Also there's an equally enlightening (albeit much shorter) interview with the author that discusses this principle.