The real question to me is: Was that even perfect susanoo? That could have simply been Sasuke's legged susanoo shaped to wrap around Kurama. Why do I say this? I say this because Madara wrapped his perfect susanoo around 100% Kurama. Naruto only had 50% Kurama (meaning that Sasuke only wrapped his around a half Kurama). We know that when Minato took the yin half, Kurama got smaller. This means that Sasuke may have been wrapping his susanoo around a smaller Kurama. Therefore, that susanoo may not have been the same size as Madara's perfect susanoo. Furthermore, Sasuke's legged susanoo was the size of Naruto/Kurama in full beast mode. This evidence leads me to think that it was actually just legged susanoo that Sasuke wrapped around Kurama instead of perfect susanoo. It's possible that Kurama simply regained his original size over time, but if he did not then my theory is very plausible.