The Invincibles were too small for F35b. A cut and shut was considered, adding a section just ahead of the bridge. But this was thankfully ditched, for being expensive and offering little extra life.
In the end the offer from the treasury was for 2 larger to replace 3 smaller. If the RN had insisted on staying small, it would have just ended up with 2 smaller, far less effective ships. The size of the QEs was (sensibly) based on what was required to deliver a predetermined capability.