I agree with the north's advantage being in numbers and resources, but I think it's more complicated than him trying to batter down Lee with it. This image of Grant throwing lives away needlessly comes from the Overland campaign in the war, and it was a complex situation with both of them probing for advantage and Grant eventually pushing him into a siege. Lee lost initiative and wasn't able to recover from that. Sherman, another Union General was able to go into the deep south after that and destroy the south's ability to fight leading to a surrender. I think strategically, he was able to out-perform Lee which no other Union general was able to do in the civil war, which was pretty different from how wars were fought up until that point.

Comments on this post lays it out in much more detail.

https://www.reddit.com/r/AskHistorians/comments/1uabom/grant...