api svid bhagavan esa
yajño me khedayan manah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
First of all this boar was seen no bigger than the tip of a thumb, and within a moment He was as large as a stone. My mind is perturbed. Is He the Supreme Personality of Godhead, Visnu?
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Since Brahma is the supermost person in the universe and he had never before experienced such a form, he could guess that the wonderful appearance of the boar was an incarnation of Visnu. The uncommon features symptomatic of the incarnation of Godhead can bewilder even the mind of Brahma.