The (max) linear aspect only accounts the space that is used for dynamically written content, not for static non-changing content.
For a 4 GB flash card, filled 75% with static content only 25% or 1 GB is available for dynamic content.
Going from a 4 GB to a 8 GB flash card with the same 3 GB static content will increase available space for dynamic content from 1 GB to 5 GB, 5 times the size while the size of the card has only doubled.
(And this is a very optimistic scenario. If the size of the static content is only a fraction of the total capacity then doubling the card size will have much less effect on the increase for dynamic content, approaching a factor of 2).
What I was referring to is, if all space available for dynamic content is doubled, will that also double the lifetime or not. There is no simple answer to that because many factors play a role.