The reason is that it is not based upon size, but rather the diptychs.
The diptychs go as follows:
5. Russia (ROCOR included)
14. Czech & Slovak
15. OCA (not universally recognized as autocephalous)
The three bolded form the officers of the assembly, since they are the first 3 in the order of the autocephalous churches to have a presence in America.
The only churches that have a presence in America are in italics.
The OCA is the youngest, and is not universally recognized as autocephalous, and therefore we fall last on the diptychs. According to most other Orthodox Churches, we should be classified under Russia, but since we (and Russia) view ourselves as autocephalous, we refuse that designation.
I think that Metropolitan Jonah and the OCA was offered the third spot, but we would have to agree that we are under the patronage and administration of Patriarch Kirill and Russia. We weren't willing to surrender our autocephaly, and therefore had to exercise humility and exclude ourselves from the officers and accept our place at the end of the diptychs.