Why do we say Brandish is important? Because she can shrink something the size of an Island. Why do we say Ajeel was really impressive? Because he can create a sandstorm that covers all of Magnolia. Natsu evaporated a lake with his body heat alone. Not with his actual attack, but with the body heat coming from just entering FDK mode. Also, the lake that Natsu evaporated is most likely
Lake Scilora, which is actually quite large.
This is the lake directly in the north of Fiore, and right besides the Fairy Tail guild. It's the lake in which the Phantom Lord guild marched through. In other words, this lake:
Now you might say that size isn't everything, and I agree. Size isn't everything. However, Natsu's body heat didn't just evaporate Lake Scilora, it also burned up Jacob. It's not just a display of Natsu having enough power to melt a bunch of magic-less water, but also an example of Natsu being able to overpower a Spriggan-level opponent.
Reminder that evaporating the lake and burning up Jacob wasn't the actual attack. The actual attack knocked Jacob out cold. Jacob didn't receive any major damage prior to getting hit by demolition fist. Serena already beat up the Ishgar Heavenly Saints
and casted 2 dragon slayer spells against them prior to using Neptune Dragon's fist. Unless
Makarov's fist or the
NaLu combo did more damage to Jacob than all of those attacks did to Jura, Demolition Fist is superior. Quite frankly, Jacob seems to be in more pain from Natsu's body heat than he was from Makarov's punch of the NaLu kick. Natsu did the
vast majority of damage to Jacob via demolition fist. In comparison none of God Serena's opponents were actually knocked out. They were all still conscious:
Even if you say there's a negligible difference between being knocked out and being beaten to the point where you can't retaliate, there's still the power-level difference between Jacob and someone like Jura to account for. Jacob is a Spriggan 12, and therefore he should be more powerful than any of the Wizard Saints. Do we really have any reason to assume that Jacob's durability is weaker than someone like Ajeel's or Brandish's? No. He didn't get taken out by an attack less powerful than attacks that took them out. He probably took a punch from Natsu better than Ajeel did.
Let's set aside FDK for one moment, and go back to the whole Jacob vs Wizard Saint thing. It took at least 3 hits for God Serena to take out someone like Jura (At least 1 for before he activated DS magic, and 2 afterwards before he used Neptune DS). Even if you want to say that Jura is stronger than a Spriggan like Jacob (???), it's not like we don't have a feat of Natsu oneshotting someone on Jura's level. Tenrou Bluenote was a match for Gildarts, which is where GMG Jura was hyped to be. Unless Jura massively outgrew Bluenote over the 1 year timeskip (no evidence of this), they should be around the same level. There is zero doubt in my mind that if Natsu can oneshot Bluenote with one of his basic spells, that he wouldn't be able to just oneshot any of the Ishgar Heavenly Saints with an FDK spell - especially since that the FDK spell can take out Jacob. Whether or not he can take out multiple people would just depend on AoE, which is more than sufficient.
So when we put this all together, what do we have? Natsu's attacks in FDK has
larger AoE, and did
more damage against a
stronger opponent. If this doesn't meet the requirements for what makes an attack stronger than another then I don't know what does.