A couple things from the way I understand magic in the universe, the example of fireball: some spells will come with a minimum quanta because in order to make that spell that spell you need to add mana (energy) to a minimum level to give it certain attribute(s). This is why you can always cast at higher level, using way more energy, but not less. There are other fire spells at a lower level, such as [Insert 1st level spell with fire attribute] but it lacks the explosive attribute. The explosive attribute requires way more energy than just creating flames just to acquire so a closer example would be more like, why can’t I make it ice ball? Change the elemental attribute (much easier to change and lower costing. Probably just a restriction of game mechanics than world restriction, but potentially an issue with attribute matching)
So I regards to the difference between fireball and [Insert 1st level spell with fire attribute] level , you’d start off casting [Insert 1st level spell with fire attribute] and you can increase it’s power until you may as well just spend that energy on the explosive attribute. Though if you didn’t want that explosive attribute you still could cast [Insert 1st level spell with fire attribute] with the same energy as fireball. I’d also assume the value coefficient for spells changes as you scale and the more efficient use of high energy costs would be the high level spells. I.E. you get more value in damage from 5th lvl fireball than 5th level [Insert 1st level spell with fire attribute].
Another way to imagine it would be like summoning. I cast “summon frog”. I get a frog. Why can I not get a smaller frog? Because this is the size of frogs. But if I add even more energy to it I can add a growth attribute to the spell, so when I cast summon Giant frog, I get a big boi frog. If I try to reduce it, I get a French delicacy rather than a summoned frog. Alternatively I could do, summon tadpole and get something different, and weaker, but still a frog-ish attribute
Edited for clarity, removed cantrip.
why can’t I make it ice ball?
As an aside, the last 5e character I played before switching was an Order of Scribes wizard, which can do exactly that.
So I regards to the difference between fireball and firebolt, you’d start off casting firebolt and you can increase it’s power until you may as well just spend that energy on the explosive attribute. Though if you didn’t want that explosive attribute you still could cast firebolt with the same energy as fireball.
You can’t though, Firebolt is a cantrip, you can’t upcast cantrips, they just get more powerful as you level up. They don’t even use spell slots, it’s just a different empowering mechanic entirely. Guess what though, what you’ve described is how it works in GURPS.
