Most of the Gītā moves in a steady line of eight syllables, the metre of storytelling and teaching. It is the rhythm Kṛṣṇa uses for almost everything he explains.
Then Arjuna sees the universal form, and the lines get longer.
From 11.15, where he begins to describe what he sees, the verses stretch into a heavier, longer line, the metre of hymns. Count the long verses in chapter 11 and there are 36. 28 of them are Arjuna’s. Outside chapter 11, the whole Gītā has only 19 more.
Nobody tells him to change rhythm. Awe does it. The things he sees will not fit in eight syllables, so his speech grows to hold them.
Awe didn’t silence Arjuna. It made every line longer.
Going deeper
Chapter 11 is the vision of the universal form, and 11.15 is where Arjuna begins to describe what he sees. The verse before, 11.14, has just shown him overcome with amazement, his hair standing on end, bowing his head with folded hands. Then 11.15 opens his speech: he sees in Kṛṣṇa's body all the gods, the hosts of beings, Brahmā, Śiva, the seers and the shining serpents. 11.16 goes on: arms, bellies, mouths and eyes without number, and no end, no middle and no beginning to be found. The story listens to the sound of this speech. From here the lines lengthen into a heavier measure, and the count above is a guide to that. Nothing in the text tells Arjuna to change his rhythm. The verses simply stretch as he tries to say what he sees. Read the scene in chapter 11, and what the Gītā says about devotion gathers related verses.