HIee....
Here i have a qiuck sketch of the possible reason...
Normally Lenses that changes in its physical length when zoomed
tend to have a smaller aperture at its longest zoom position. Those
with constant apertures normally have wider front lens element ......
and internal zoom/focus.....
However, there are lens like the Tamron 200-400mm but its at f5.6
fixed aperture....obvious as its like taking the worst case with respect to
the front lens element.....
Even a 200mm F2.8 + 2x Teleconverter physically will be as long as the 100-400mm and same aperture ...i.e f5.6
Experts....out there...correct me if i'm wrong....
At the end of the day....do expect lowered light gathering ability if the physical lens extends it self like pinocchio's nose...
Remember if you take the MRT ans the train is coming out of the tunnel.....the nearer it gets to the opening...the brighter the outside light.....
rgds,
me