I didn't see how the Mokujin tanked a single slash of the perfect Susanoo's sword. Merely stalemating latter was within the Mokujin's capabilities, and yet it's not an evidence of how this fight would progress in the long run, knowing that the Mokujin didn't display a move which could damage the perfect Susanoo, while latter still has its mountain busting swords (if we consider Sasuke having Madara's feats). While Madara never displayed further Mangekyo techniques aside of Susanoo, Sasuke is known for using both Amaterasu and Enton. Former already set on fire a Bijuu sized entity and latter is able to perfectly control those flames and produce further.
The size of Sasuke's perfect Susanoo, even if being smaller than Madara's, doesn't necessarily mean that its offensive power pales in comparison to Madara's, seeing how his PS also sliced a mountain range while armoring Kyuubi, being considerably smaller than in its stabilized general form.
Hashirama was also in Sage Mode while using the Mokujin against Madara's perfect Susanoo. We learned that Sage Mode increases the power of your techniques in general.