Oh. So you are talking about sequences. Timelines are what measure out the sequence, or you can just use them pretty much interchangeably. You said you duplicated a sequence and I could not figure out how that would cause a problem. Then you said you have backup sequences - there is no way to back up just a sequence unless you import it into a new project and save it. Not to worry, I think we have focused in on it now.
I don't have any projects that large, so I am not sure what could cause it besides effects and keyframes. Audio keyframes could easily be the culprit. I can certainly make one that large. However, there has to be some difference between the small project and the large project. Dig deeper. Or, post a copy of the two projects where I can download them and I will compare them using a text tool.
The biggest project I have has Warp Stabilization on it but the ones with audio keyframes are big also.