Why would a higher tidal volume decrease X?
Decreasing the volume of maximum air breathed in and henceforth, breathed out, means that for any given time, the amount of air taken in (and so, oxygen) is less. So, the quantity of fresh air in the lungs decreases every passing unit time. Also, with less air breathed out, the stale air is not being thrown out as efficiently as it would be with an 'increase in Y'. So, basically, moderate exercise should result in death of a person in a matter of a minute because he is typically destroying his diffusion gradient.
It's true that an increase in tidal volume would occur; but if anything it should increase the vital capacity, because it will most definitely decrease the residual air, one way or another.
