Paulbe, Minato seems to be saying it is conceptually impossible to seal the entire Fox in a seal (this is speculated to be the Death God Seal).
The jist of this seems to be the same as saying ''it is conceptually (scientifically) imposible for an alkaline to neatralise an alkaline' or it is conceptually impossible for a low density object to project stronger grvity than a higher density object''.
In otherwords Minato isn't saying he can't seal the Fox because he is weak or has a low than average chakra. He saying it is 'scientifically impossible to seal the entire Kyuubi in the Death God seal because the seal simply doesn't work that way the same way a car cannot run without petrol.
The fact that it is conceptually impossible to seal the entire fox can be potentially routed in various logic i.e. the death god container is only a certain size and can't fit the entire Kyubii's chakra( in otherwords both the sheer scale of the Fox's chakra and the relative smallness of the Death God are to blame.
Or it could be based on science i.e. the same way an electrical device would not work under water so too would it be impossible for the entire foxes chakra to be sealed in the Death God.
Whatever the case Yondaime was not saying he was too inadequate to seal the entire Fox in the Death God seal. What he was saying was that it was SCIENTIFICALLY Inconcievable to seal an entire fox using the Death God Seal. The seal simply doesn't work like that.
I think this is what you were trying to explain.
ON a side Note
Not much is know about the original seal used to seal the full powered Kyuubi inside Kushina.
There is a chance Minato could possibly have used the original seal used to seal the entire Kyuubi inside Kushina-for naruto as well Naruto but simply choose not to because it would be more prudent to permanantly seal half the Kyuubi inside the Death God in order to:
(1) Somehow disrupt Madara's plans (perhaps miniizing chances of a jyuubi ressurection)
(2) Minimize danger if the Kyuubi is accidentally unleashed from Naruto (i.e. a Kyuubi with half its stregth sealed in a death God is less dangerous. Please Note Konoha ninja were able to make a half assed attempt to drive the full powered kyuubi from Konoha, how much more an only half powered fox (albiet they had no way of coping with the kyuubi's cero and prbl would have lost eventually).
Both these are simply speculation on my part.