Weird encoding problem - filesize

[dustbug]

Heya, I am trying to encode Futurama eps to Xvid & I am having a problem which I've never experienced before.
I've been using DVDX; I load up my IFO, select which ep I want to encode, & put in all the settings I want. It encodes fine, giving me a file size which I was after for 1 ep. Ok so then I move on to the next ep (it is exactly the same length) - I use the exact same settings as the first ep, encode it, but the resulting filesize will be completely different, (ie 40mb bigger or smaller etc).
I then tried using Divx, instead of Xvid & the same thing happened. Getting somewhat annoyed, I then try to load up all the continuous VOBs to encode in 1 big file, with the intent to split up the big file into individual eps. After it has encoded, I split the file into eps, but the eps are still vastly different sizes (even tho they are exactly the same length & all have been encoded in 1 hit, obviously with the same settings).

Does anyone know what is going on?
Is there a program out there where I can load up an IFO, select the ep I want to encode & then put in a file size I want that ep to be coded to? (& it will actually be encoded to that size, rather than all differing sizes)
Please help, I am stumped & extremely frustrated hehe

[scottws]

I have somewhat limited knowledge, but I think I can adequately explain what's going on.

From what I understand, the way DivX and XviD work is that they save space by recording little of the actual frame. Each DivX video will have a series of keyframes, which do in fact store all of that frame's image. But for every frame in between keyframes (300, if you use DivX 5.05s default setting), the only information stored is the difference between the frame being encoded and the previous keyframe. If certain parts of the frame being encoded are the same as the previous keyframe, then those parts require little information to describe them; just enough to say that it is the same.

In short, it really just depends on the video being encoded. DivX and XviD are variable bitrate, meaning the number of bits to encode each frame can vary depending on what has changed from the previous keyframe. Therefore, unlike a constant bitrate codec, predicting filesize can be difficult.

Essentially, no two different videos encoded with DivX or XviD at the same settings will have an identical file size. Every video has a different compressibility ratio, depending on how much changes from frame to frame.

It sounds like your second video is much more complex, or changes a lot from frame to frame and is therefore less compressible.

40MB seems like a lot though. Are you sure you used the same settings for both vids? Did you multipass? Multipassing helps keep the average bitrate close to what you set.

[dustbug]

Thanx for the reply
In answer, yep, Iused exactly the same settings, & multi passed

Its weird, I never had this problem when doing Simpsons eps DVDrips, using the same method. Each ep came out within 1mb file size of each other