Well all of the Wizards you are giving examples for have high Convictions giving them extra Consequences to call on, so perhaps they were taking a few of them to offset the cost of their spells.
Also you might consider what constitutes a scene. One of the neat things that I love about FATE is that Stress isn't a hit point or damage mechanic, it's actually a pacing mechanic. When Harry or any other character in the books fight against impossible odds and after they have taken out a bunch of guys, are all out of juice and still surrounded by enemies, it's time for a new scene even though it is the same battle. Harry gets a second wind, he digs deep and even though he's hurt, broken, and bleeding he pours on the magic and the bad guys start burning. Thinking of Stress and the times to get rid of those checked boxes as chapter, act, or commercial breaks, or even that dramatic second wind before the shit goes down instead of traditional scenes can really make the Dresden Files game sing.
Lets take your Summer Knight examples, I don't have the book with me and it's been a few years since I read it so sorry if this isn't how it goes but I'll use my version of the action of the spells you described to give my example. So those first four spells sound like they were in one scene and then the next two were in a brand new scene of the battle where Harry gets to drop all the stress he took and start fresh, though maybe with a consequence or two because of the previous scene's spells power levels.
Those first four spells dealt with minions and they were maneuvers, blocks, and attacks. They were cool, they were powerful, and they show that Harry isn't a chump and can walk all over a bunch of dangerous things from the Summer Court. Which is great, but then the chapter probably ended and he then had to deal with the heavy hitters of Summer, Talos, Slate, etc. so it is a brand new scene, and he erases all that stress he just took powering his magic spells that got him to the heavies.
Now Harry confronts Slate and Talos, he whips out the major mojo and takes a bunch of Stress and probably some new consequences to do it, he puts down Slate and the rest probably with a block and then smites Talos with fire since he was the only one powerful enough to break through it. Now he's spent and facing off against the Summer Lady and he is all out of Stress and Bubblegum.
But luckily he is a Wizard and he plans ahead, and as he is unleashing his Contacting Attack on Aurora he tags all those free Aspects he maneuvered onto the game in earlier scenes with a few Resource rolls for "Trip to the Hardware Store", and "I Have Pizza Express on Speed Dial" and the Lore roll that got him "Pixies Will Do Anything For Pizza", he then hits Aurora with his mighty army of box cutter wielding pixies and does enough stress to take her out, for he is truly the all powerful Za Lord!
Or at least that's how I see it playing out.
Also keep in mind Fate Points to make powerful spells even more powerful, Harry had been beat up all through Summer Knight and as usual he had also taken a bunch of compels leaving him with a lot of Fate Points for that climactic battle. Since Evocation attacks use the targeting roll to hit and any extra shifts translate to damage, well with a lot of Fate Points you can invoke a lot of aspects and those +2 bonuses start to add up quick when it comes to damage, even if the actual Evocation Attack is only a low Weapon damage.