I agree
It may be due to Sasuke's lack of expierience with using Perfect Sussano as it was his first time...
but it also can be due to Sasuke had to limit his size to coat Naruto's Kurama shroud... If he used 100% size Perfect Sussano then maybe it would be just waste of chakra and maybe even Kurama/Sussano fusion couldn't work(They may have to be same or around same size). Also Remember that Sasuke needed Senjutsu to hurt Obito... that why his Sussano shroud needed to be in direct contact with Kurama's shroud which Naruto filled with Senjutsu.
@smd
On topic: Honestly I believe that Enton coated 100% full size Perfect Sussano could be like that:
100% BSM Naruto < Enton Coated 100% Full size PS> 100% BM Naruto < 100% Full Size PS>=50% BSM Naruto=50% Enton coated PS> 50% BM Naruto=50% size PS
That what I think

feel free to disagree