48, Accounting for Acquisitions and Mergers, proposes to restrict poolings to combinations in which "none of the combining parties is more than 50% larger than any other enterprises [that] is a party to the combination unless special circumstances prevent the larger from dominating the other(s)." This rule translates to a relative size of 40% to 60% for the pooled entity.
In fact, the value of the shares exchanged often is far above the pooled company's share price at the announcement date.