I would prefer an intuitive explanation rather than a deep mathematical dive
No equations, I promise! 😉
Why do newer engines have such wide fan blades?
To answer this question, it has to be understood how the blades of a jet engine work (regardless of where they are mounted, i.e. on the compressor, on the turbine or on the stator). In particular, their main task is to bend as much as possible the airflow that passes over them. The following picture (source) shows how the airflow's velocity changes due to the blade's presence:
Being $\beta_1$ the angle of the airflow entering the blade and $\beta_2$ the angle of the airflow leaving the blade, a good blade design is where their difference (normally termed $\epsilon = \beta_1 - \beta_2$) is as big as possible, theoretically just equal the angles of the leading and trailing edge. To achieve this, the distance between two blades (called pitch, s) must be within a quite narrow value, around 1 to 0.5 the chord, c of the blade. Intuitively, this can be explained as a sort of "channel effect" that is created when two blades are close to each other and that helps the flow to follow the blade profiles. If the blades are too far apart, this channel effect vanishes and their efficiency too. Anyway they do not even have to be too close (pitch<0.5), otherwise a "blockage" effect results.
Now to answer your question: if the blades went out of the rotating axis just straight with a constant chord, then their pitch s would increase more and more moving away from the axis, losing that "channel effect" and efficiency. Increasing the chord of the blades proportionally to the distance from the axis, simply keeps the pitch within the optimal value.
long, thin wings are more efficient than wide stubby ones
Correct. Simplifying to the extreme, a blade or a wing work pushing air downward and receiving as a consequence, by virtue of the third Newton's law, a net "push" upward, that we call lift. Lift is proportional to the product of the speed at which the blade/wing pushes the air downward and to the mass of air being pushed. By simple energy considerations, it can be demonstrated that this process of generating lift is more efficient if a lot of air is pushed with a small speed. This is basically why a blade or a wing becomes more and more efficient the wider it gets and that's why helicopters and gliders tend to have lifting surfaces with very long span (technically they generate less induced drag).
why is it that... engines have gotten progressively larger in diameter?
The reason should be clear now: the longer their blades are, the more efficient they get. But why then jet engines didn't have a wide fan since the very beginning? Technological limitation: a rotating mass experiences a centrifugal force proportional to its distance from the axis of rotation and to the rotating speed squared. Just as a comparison, a jet engine rotor with its 15'000 rpm sees a centrifugal force which is 2'500 times higher than a helicopter rotor rotating at 300rpm! Structural strength limits the maximum diameter of a jet engine to a couple of meters and this structural limitation was even more strict back in the early days of jet engines: with the evolution of material technology the diameter has become increasingly bigger... anyway we are never going to see a jet engine with a fan as big as a helicopter rotor. Technological evolution also explains the fancier shape of the blades which now can be driven not only by mechanical requirements (it has to be straight to resist centrifugal force, full stop) but also by aerodynamics requirements.
why is it that... engines have gotten... progressively fewer blades?
This is not completely true. An old (1974) CFM56 had 24 blades on its fan, just like a modern (2003) RR Trent 900. An old RR Convey (first turbofan to enter service) had 20 blades, only 2 more than an ultramodern (2006) GEnx. So the number of blades is more related to the producer's in-house technical choices than to an historical evolution.
As promised, no math 👍