From one of the papers:
"Similar considerations hold among computational objects. For small enough objects and transactions, the cost of accounting and negotiations will overwhelm any advantages that may result from making flexible, price-sensitive tradeoffs. For large enough objects and trans- actions, however, these overhead costs will be small in percentage terms; the benefits of market mechanisms may then be worth the cost. At an intermediate scale, negotiation will be too expensive, but accounting will help guide planning. These scale effects will encourage the aggregation of small, simple objects into "firms" with low-overhead rules for division of income among their participants.

Size thresholds for accounting and negotiations will vary with situations and implementation techniques [16]. Market competition will tune these thresholds, providing incentives to choose the most efficient scale on which to apply central-planning methods to computation."

http://www.agorics.com/Library/agoricpapers/aos/aos.3.html

-Andrew