usually its
1) price
2) size
3) hearsay
The #1 thing you will notice is the change in capacitor film technology, aka improvement. They can make thinner films more reliably that do the job so the caps are smaller, and better electrolyte.
With old machinery you can guarantee the film is durable when its above a certain thickness. With newer processes, the film can be made thinner better. Possibly it has more dielectric resistance and stuff like that too. Or it might be the same stuff and just tested better to give you the 'reliable spec'.
Lets say you have a small instrument that uses a particular cap. If the big ones can use it, use it, then you can get more volume discount. They have to balance between quality and cost. It still came out really good. Its just like having a internal E-series they tell the designers to use, based on their manufacturing stockpile.
When they are too big it might stress things too, during turn on, absurd values require special management circuitry.
To me that one is kinda a loose spec. Like a ships anchor. It kind of depends on what the equipment is plugged into. But there is a minimum size that you can determine looking at ripple, theoretically, if you consider just a clean 60Hz sine wave with no distortion with some stable load plugged into the output of the equipment with some particular settings, at a standardized line impedance for the generic spec (automotive or residential/office like). But when you start having generators an heavy equipment nearby things can get very complicated I think.
And parallel caps.. that has to do with
1) better form factor, its how you get a 'rectangle shape' instead of a 'tank shape'
2) lower ESR, its parallel after all
3) possibly more economical if you can pick from your companies "series". They could also pick from values they noticed statistically are in stock more or easier to source from a alternative manufacturer
4) because of #2, it probobly also filters RF better, since the caps are more ideal. That means maybe they can get rid of some other component, like a lowish value capacitor.. so really its possible that you end up with the same amount of capacitors because it turns out you need another one if you go with a big one that has higher parasitic values
Find the best lowest ESR capacitor you can find at some value, and compare it to the ESR you can get with parallel capacitors of equal quality but smaller size. I am thinking the first one can be alot bigger, the second one, after the resistor, you want that one similar