https://en.wikipedia.org/wiki/Lockheed_C-5_Galaxy
(I think the typical reader will have little intuition for the cargo capacity of the C-17, so the more instructive comparison looks at the most capable operational US plane.)
http://www.avionale.com/wp-content/uploads/2015/03/Lockheed_...
c5 Galaxy is massive.
Interestingly, Boeing 747-8F is even bigger, heavier, and carries more load.
Antonov 225?
Well, it can carry the same load as both of those planes combined, which really put things into perspective.
Also note that (according to Wikipedia) the capacity is 250,000kg - so 275 tons, or 551,155 lbs.
The C-17's cargo capacity (again, from Wikipedia) is 77,500kg - so 85 tons, or 170,858lbs. That looks a lot more like a difference of 380,000lbs than 300,000lbs to me, which, with some journalistic licence, could surely become "almost 400,000lbs more". You may well be correct about the sensationalism aspect, and here they are leaving some of that on the table.
So I don't really know what's going on. Like I said, functionally innumerate... (Though of course, calculation error at this end is always a possibility! But nobody's paying me to do this, so I'm going to go with my gut here.)