Well, it's fun stuff, especially when you consider the various complications involved! For example, just consider, for example, the discussion of a 50mm lens at f/2.8 and a 200mm lens at f/2.8 -- the aperture opening with the 50mm is smaller than a 35mm (full frame) sensor, so the light can just shine on in with no noticeable distortion. There's a reason why the 50mm f/1.8 lens is cheap and yet delivers great IQ at f/2-f/2.8. it's because the lens delivers light that doesn't need to be "messed with", no engineering optics to make it more expensive!
But then move up to the 200mm f/2.8 and you will see the challenge: the aperture of the 200mm has to be wide enough to deliver all that light, but not to the sensor, the camera frame is not that large, not to mention the sensor! So for a telephoto lens, optical engineering and elements have to go into place to compress all that light, and then if you want a good lens, that work has to be of premium quality, to avoid distortion from all that compression. In fact, even very good lenses can occasionally show up with softness around the edges and in the corners...possibly a result of that compression, I don't know, 'cause I'm not an optical engineer!
And then, go even farther, to say the 300mm f/2.8 or the 400mm f/3.8, those things are huge beasts! And a big reason for this is that aperture keeps getting physically wider, and so the body of the lens has to be wider, and then the internal optics have to go to work to take the light delivered by that aperture and to "boil it down" so that the signal passes through the lens mount and onto the sensor!
And so, those who have delved into the "big lenses", whether it be the f/2.8 Canon L lenses or the high-end Nikon lenses have an answer as to why 1) those lenses are so freakin' huge, and 2) why they can be so freakin' expensive (which only factors in the optical engineering at the back of the lens, not that that goes into the light-collecting qualities at the front of the lens!