A "big number" is a large quantity or figure, either in math or everyday talk. It suggests size or impact that feels impressive, hard to count, or hard to grasp.
If you look at my Twitter feed it is 99% links, but 1% is me responding and 1% of a big number is a big number. Guy Kawasaki
Infinity...is used in physics simply as a shorthand for "a very big number.”. Victor J. Stenger
I know if I stopped hosting 'Wine Library TV,' we'd probably lose 75 percent of our audience, but the remaining 25 percent is still a big number. Gary Vaynerchuk
The largest portion of our state budget is Health and Human Services. It will grow by approximately 500 million. A half a billion dollars is a very big number. Mitt Romney