Sutra:
14. vAcyap prabhAvam pOlanRu vAcakap prabhAvam.

Meaning:
The greatness of the manthra that speaks of the Lord is not the same as the greatness of the Lord Himself (that is, it is considered greater than His).

Sri PBA Swami's Sarartha Deepikai:
Till now, it was shown that the Thirumanthra is to be considered higher than other manthras. Now it is being said that it is higher than even Him.

vAcya: the meaning of the word, who is Him
vAcaka: the word, which is Thirumanthra

The greatness of the Thirumanthra is not equal to that of the Lord of whom it speaks. It is considered greater than His greatness.