> Hayek never explained how global corporations, the
> outputs of which matched that of a small or
medium-sized economy were capable of organizing the
requisite information, but the planning department of 
> such a government would not be up to the task.
> Michael Perelman


The difference is that an economy engages in
production, exchange, and consumption, while a firm
engages only in production.

A completely centrally planned economy would need to
plan the consumption of each individual, whereas the
maximization of utility is based on individual
preferences which change and cannot be known to the
central planner.

Central planning can work in a very small economy
which uses market prices for its inputs and outputs. 
For example, a family is a micro-economy which is run
with central planning and communist principles. But
the greater the number of people in a family, the less
effective is the central planning.

If we model the interactions of n people as n squared,
the complexity grows by the square of the number of
individuals. Hayek's concept of decentralized
knowledge becomes ever more applicable the greater n
is.

A huge army can be centrally planned because the
consumption by the individuals soldiers is not a
priority; the main desired consumption is the
destruction of the enemy.  

A large firm is effective because consumption depends
on the unplanned demands of its custormers, and the
prices of inputs and outputs are set outside the firm.

Fred Foldvary