So infinity-1 is smaller than infinity?
However, these sets have exactly the same number of elements (i.e. "infinity-1 is the same size as infinity") -- I can pair 0 with 1, 1 with 2, 2 with 3, and so on, so that every number in the first set is uniquely paired with a number in the second set, and vice versa.
Arithmetically, I can write `f(x) = x + 1` to go from the first set to the second set, and `g(x) = x - 1` to go from the second set back to the first set, and "clearly" we don't lose any information performing `f` followed by `g` -- no two elements land in the same place in either direction.