Treating aoe damage as equivalent to multi attack damage is IMO one of the largest sins in D&D analysis. Ask yourself, is it better for the fighter to do 25 damage to 4 targets or is it better that he can apply that 25 damage to one enemy up to 4 times (enemy hp permitting).
Again, it's really, really going to depend on the table.
After all, as was mentioned, doing 25 damage to 4 targets sets up 4 potential deaths rather than 1 or 2 depending on how many HP the baddies have. Additionally, if your DM plays the monsters as intelligent, it's not unreasonable that 4 targets run away after all being damaged like that.
Or, ask yourself this. Which ends a fight faster - two fireballs or a fighter? And, as was mentioned, in any encounter where you have lots of smaller targets, rather than one big one, fireball is going to be far more effective.
So, yeah, you have to calculate total damage. In your example (and note, 25 damage/hit from a fighter is HUGE - we're talking GWF hitting every time to get that kind of average) if the enemies have 26 HP, you waste most of your damage plus 2 of your attacks. With fireball, I only need to make one action to deal the damage. There really isn't any wasted effort because the cost is so low.
For that fighter to do that, before 11th level anyway, he'd have to action surge and then attack 4 times, with two of his attacks mostly being wasted. An action he cannot repeat. The wizard drops a fireball, kills all four targets plus potentially more if they're in the area, and he can (after 5th level anyway) do it again and probably several more times before needing a rest.
Like I said, track the damage. Heck, even only track the actual damage - don't track any extra damage that goes over the monster's HP - and the wizard is so far ahead of the fighter that it doesn't matter how much you buff the fighter. At best that fully buffed fighter with GWF and a flaming sword is just equal to the damage that that wizard is doing. Which is what it's supposed to be. Fighters aren't supposed to be the primary damage dealers.