I'm sorry, but I just have to say this... where the hell are you pulling these numbers out of for your calculations? Honestly, looking it all over it comes across as an arbitrary set of numbers put into a calculation to create an implied power variance that in truth is no where near as out of balance as you try to make it out to be.
Lets look at this from an ACTUAL numerical standpoint shall we. Both starting at Lv6 with 16 in their key attribute (Int & Str respectively)
Fighter gets 2 attacks at +9/+4 (6bab+3strmod) and deals [w]+3 base damage per hit... You've got at least 7 feats to work with, I'm only going to factor the 4 fighter bonus feats to get the best impression of the classes capabilities.. Depending on if your going TH or TW you will either have Power Attack or TWF as one of those feats... odds are your going to have Weapon Focus & Weapon Spec as two of the remaining feats... so using that as a base we've got two sets of results.
TWF - 3 attacks at +8/+3|+8 and deal [w]+5/[w]+5|[w]+3 damage netting 3[w]+13 damage per round
TH - 2 attacks at +4/-1 ~ +10/+5 and deal [w]+6~12/[w]+6~12 damage netting 2[w]+12~24 damage per round
Not to mention, you still have 3 more non-class feats to further improve that along with any number of enchantments you could add to your weapon(s)...
The wizard with their 3rd level spells is likely using Fireball which they can cast 3x per day at 6th with 16 Int and has 4 feats, 1 from class. Given that you only have up to 3rd level spell slots you're not really going to be able to use any damage increasing metamagics such as Empower or Maximize spell. So your wizard bonus feat choice won't really factor for much at 6th level, at best it'd probably be craft wand to craft low cost wands of fireball with...
Fireball - deals 6d6 damage in a 20ft radius... That gives you a total damage range of 6 (1 enemy, all 1s) to 1,584 (44 enemies, all 6s)... realistically you'll only encounter an average of 5 enemies in radius and an average of 18(6x3) damage each for a net average of 90 damage.
However after 3 rounds that damage is reduced down to the capabilities of 2nd & 1st level spells and any magic items they may have. Assuming they took craft wand they'll have up an extra 50 rounds of 5d6 fireballs giving them an average of 15(5x3) damage per enemy for a net average of 75 damage across an average of 5 enemies.
So at 6th level you've got Fighters dealing up to 3[w]+13 /2[w]+12~24 damage per round vs Wizards dealing 6~90 damage per round for 3 rounds and 5~75 damage per round on subsequent rounds. When put into situations where AoE against more than 2 foes is a factor then yes wizards are indeed exponentially stronger, but in 1ob1 or 1on2 a fighter performs just as well.
If the supposed exponential power growth of wizards is figured via versatility, then a similar set of comparisons can be made for a fighter who again can repeat their action round after round endlessly whereas a wizard has a limited resource. It's not until late game when the limited resource of spell slots becomes abundant enough per spell level as to become negligible.
If on average a fighter were to miss half (bad case scenario) of their attacks and combat were to last for 5 rounds per encounter then we are looking at an average of 8[w]+32 (TWF) or 5[w]+30~60 (THF) damage per encounter for fighters and 84 damage per enemy per encounter for wizards... If we assume the fighter's using a 1d6 weapon then we've got +24 average for [w] per encounter for 68 (TWF) & 54~84 (THF) damage per encounter... and all of this is before even factoring in critical hits.
So the way I see it a Fighter and a Wizard are really on almost equal footing at lv6... The simple fact that a fighter's power level can be almost even with a wizard with 3rd level spells at 6th level is proof enough in my eyes that the power curve for wizards is no where near as steep as it is generally accepted to be, nor is the fighters progression truly linear either.
Comparing at 20th level and even a single hand fighter with 16 str exceeds the strength of an empowered maximized fireball with 160 damage per round vs the 90 damage of the fireball... If you use metamagic cost reductions you can get that fireball up to 180 with by adding Enhanced to make it (20d6)x1.5... if your cost reductions keep that Enhanced Empowered Maximized Fireball below 9th level slot then you could bump it up to 210 by adding in Fiery (20d6+20)x1.5... but that's entirely circumstantial and would involve using non-class feats &/or prC to accomplish... so we're looking at 90-100 within the confines of the wizard class alone.
Also I really have to correct ya'll on that claim of wizards being "quadratic"... you realize if wizards were quadratic their power would peak and then decline at the same rate it originally grew... so a "quadratic" wizard would be strongest at level 10 and weakest at levels 1 and 20... considering that they do NOT ever lose power (no one does) the term ya'll were after was exponential... Fighters I would actually argue to be cubed as they start with an exponential growth that eventually tapers off to a logarithmic growth, thus they are cubic. Properly graphic a fighters power will create an s curve. Here's a resource for ya'll to observe how different types of graphs curve, as you'll notice Quadratics create a parabola, not a simple curve, and cubed (cube root) start exponentially then curve back off again.
http://sosanko.weebly.com/uploads/3/7/4/1/37411349/1285243_orig.png