Some animation companies don't even use scripts from what I've heard, they have dialog that they put on after the animation is all done.
That would be economic suicide for an animation studio. Animation is labor intensive and expensive to produce. While it's theoretically possible to animate everything and add lip syncs afterwards, that wouldn't take into account character body language and responses to dialogue, which no one is going to re-animate unless they have to.
I work in animation. Yes, scripts are typically longer than the finished show (I'm referring to television), to give wiggle room. Voices are recorded before a single line of animation is drawn. Then it's edited together with the storyboard to produce animatics, at which point scenes are cut and finagled, nipped, tucked, and dialogue rewritten numerous times.
Once the animatic is done, it's locked, and the timing is done, including each and every mouth position, written down on exposure sheets frame-by-frame at 25 or 30 frames/second.
The final thing may bear little resemblance to the original script. I'm right now in the middle of producing an animatic based on my own script, which is now on its approximately 20th rewrite AFTER the script was already rewritten 3 times and approved by the producer. Every studio and producer has their own methods, and depending on deadline, they may require scripts closer to the target length. That was the case when I last worked in a big studio (before the computer age when it was even MORE expensive to produce).
For feature-length animation there's a lot more pre-production work done on stories, scripts and storyboards before final animation proceeds. And it's true that they might start with either a rough script or a detailed outline, which is then worked out on a huge storyboard, with final dialogue written last. I wouldn't know, since I haven't worked in such an environment. But I've worked on shorts which followed this more ad hoc sort of development. The bad news for scriptwriters is, that's all done in-house, not from spec scripts. The good news is that many TV series do commission freelance script writers.
I've also searched online for sample scripts for some of the big studios' features and TV shows, but haven't found any. You can find fan-typed transcriptions of some of the Pixar scripts, but that's dialogue only.