The way Minato worded his death against Kurama and his reign as the 4th Hokage leads Hashi and Tobi to believe that he is inherently not very powerful. Hasirama was able to completely decimate Kurama and made fighting him look like child's play. Now Tobirama most likely wouldn't make such quick work of Kurama but he would still defeat him nonetheless.
This is however taken GREATLY out of context. Minato used a fair amount of chakara simply maintaining Kushina's seal during birth. He was then forced to fight Obito, a shinobi with a S/T jutsu that surpassed his own. After all this he then was forced to battle Kurama all while protecting the village, teleporting a massive TBB and Kurama himself away from the village consuming an inconceivable amount of chakra. Kakashi was only barely able to teleport the Hachibi WITH the assistance of Kurama's chakra and a fully regenerated chakra pool. However the way he made it sound was as if he was simply too weak to defeat Kurama. Orochimaru aslo impies that he and many people in the village believed that Minato was not fit to be the Hokage, again lowering his perceived power in Tobi and Hashi's eyes.