The disk space requirement is shown as "at least X space required". Because the result of Check functions can change at any time (including after this wizard page is shown), Inno errs on the side of caution and omits any files with Check conditions entirely from the calculation (i.e. it shows the minimum disk space requirement).
You can work around this by conditioning your [Files] on Components: instead of Check:, and conditioning your [Components] on the Check: instead. This will call the Check functions only once at startup, and since their states are now known, the disk space will be shown as expected.
Of course, this requires that your actual conditions be known in advance, without requiring any wizard input.