Do you think they would have made a much bigger dent in the awards?

Ratatouille: no. It's still an animated kid's film, and I don't think the Academy can look past that.

Zodiac: possible. Though it's very much in the same line as No Country and Blood, so it may be overlooked just because of that.

Zodiac- Yes
Ratatouille- No

Ratatouille would not be able to overcome the animated bias no matter what time of year it was released. Not in a year as strong as this one at least. Evidence: Toy Story 2, which also got VERY heavy critical acclaim (an 87 score at Metacritic, though not as high as Ratatouille's 96, is still really damn good), made more money at the box office ($245.9 million raw, $330 million adjusted for inflation), and won the Golden Globe for Musical/Comedy.

Zodiac could probably score a few major nods with a later release date. It clearly played out really well with the critics, as the only two films that made more end-of-the-year top ten lists were No Country for Old Men and There Will Be Blood. The biggest problem with that film's campaign is that its early release date has left it forgotten. If it had been out closer to nomination time, it would have been on more minds and probably would have snagged a number of nods because then Paramount's campaign actually would have had the efficiency it intended to.

Zodiac might have more of a shot at getting the 5th slot in screenplay and some technical awards, but it still would be a weak contender.

I actually think Zodiac's early release helped it. Instead of being stuck with all the other contenders, it was arguably the only Oscar-worthy film until June, and got to be the only major critical success of the timeframe. As a result, when awards time came around, it didn't get lost in the crowd, but instead has become the early-year-late-candidate. It may not be enough to get it into any major categories, but I don't think it would've fared much better otherwise.

Ratatouille, sadly, never had a chance, no matter when it was released.

Ratatouille should, but won't.