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, firebolt for instance, 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 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. 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 firebolt.

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