So I do not have additional sources from the game designers, however, I would like to point out a flaw in the "balance" theory.
It's not clear that spontaneous casters are actually balanced with the prepared ones.
Here's a discussion on that for the 3.5 rules.
Even in Pathfinder, Sorcerers & Oracles are generally considered weaker than their Wizard & Cleric counterparts. Here's a link to one ranking of the classes, but you can easily find many more. Most of them don't even put Wizard and Sorcerer on the same tier.
From a historical perspective, AD&D, the pre-cursor to D&D 3.0 did not have Sorcerers or Oracles at all. In fact there we no Charisma casters in AD&D. So 3rd edition just created these classes and gave them a different progression.
If you look at some old pages, the Sorcerer spells/day progression is literally unchanged since 3rd edition came out. So this difference has existed since the printed inception of the Sorcerer.
A little more hunting gives you some spell progressions from the AD&D Wizards courtesy of this guy. The AD&D list was very similar, you'll notice that level 7 is exactly the same, but there's also no formal Cantrips is the first printing of AD&D. Wizards also max out at 5 on each level instead of 4. Though that 5 starts at level 13. The 3.0 version basically just smooths out the progression (notice 8=>9 change), but Pathfinder is still holding on to the same progression as 3.0.
I strongly suspect this is all just an artifact of history.
Given how little the Wizard numbers have changed since 1992, designers have clearly been using numbers from previous editions as the baseline. And who wants to change these numbers? They're so ingrained in the fabric of the game that I doubt anyone even so much as experimented with them after the original printing.