The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
16. And on account of the supporting also (attributed to it), (the small ether must be the Lord) because that greatness is observed in him (according to other scriptural passages).
And also on account of the 'supporting' the small ether can be the highest Lord only.--How?--The text at first introduces the general subject of discussion in the passage, 'In it is that small ether;' declares thereupon that the small one is to be compared with the universal ether, and that everything is contained in it; subsequently applies to it the term 'Self,' and states it to possess the qualities of being free from sin, &c.; and, finally, declares with reference to the same general subject of discussion, 'That Self is a bank, a limitary support (vidhriti), that these worlds may not be confounded.' As 'support' is here predicated of the Self, we have to understand by it a supporting agent. Just as a dam stems the spreading water so that the boundaries of the fields are not confounded, so that Self acts like a limitary dam in order that these outer and inner worlds, and all the different castes and âsramas may not be confounded. In accordance with this our text declares that greatness, which is shown in the act of holding asunder, to belong to the small (ether) which forms the subject of discussion; and that such greatness is found in the highest Lord only, is seen from other scriptural passages, such as 'By the command of that Imperishable, O Gârgî, sun and moon; are held apart"(Bri. Up. III, 8, 9). Similarly, we read in another passage also, about whose referring to the highest
Lord there is no doubt, 'He is the Lord of all, the king of all things, the protector of all things. He is a bank and a limitary support, so that these worlds may not be confounded' (Bri. Up. IV, 4, 12)--Hence, on account of the 'supporting,' also the small (ether) is nothing else but the highest Lord.