The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
36. And on account of the permanency of the final (size of the soul) and the resulting permanency of the two (preceding sizes) there is no difference (of size, at any time).
Moreover, the Gainas themselves admit the permanency of the final size of the soul which it has in the state of release. From this it follows also that its initial size and its intervening sizes must be permanent 1, and that hence
there is no difference between the three sizes. But this would involve the conclusion that the different bodies of the soul have one and the same size, and that the soul cannot enter into bigger and smaller bodies.--Or else (to explain the Sûtra in a somewhat different way) from the fact that the final size of the soul is permanent, it follows that its size in the two previous conditions also is permanent. Hence the soul must be considered as being always of the same size--whether minute or infinite--and not of the varying size of its bodies.--For this reason also the doctrine of the Arhat has to be set aside as not in any way more rational than the doctrine of Buddha.
433:1 The inference being that the initial and intervening sizes of the soul must be permanent because they are sizes of the soul, like its final size.