Reimported finished videos get bigger.

Question

Here's another odd behavior I see in Premiere Pro CS6.0.5. Just noticed this last night. I have a video I finished in Premiere as a H264, 16:9 HD 720p at 29.97fps .mp4 file. Looks good and plays well. I decided I wanted to add a scene at the very end, so today I loaded it back into Premiere. Lo and behold, the video has enlarged to a much bigger size!! Since it still fills the whole 16:9 area, it looks like half the image has been cropped off around the outside and the remainder enlarged to fill the screen. And if not fixed, it Exports this way as well.
Fortunately, it is easily fixed by going into Effect Control, Motion, and then Scaling it back to normal size, but that's extra work and slightly slows down any renders.
Usually when I finish edit a video, I don't have need to bring it back into Premiere, so when I saw this for the first time today, it really puzzled me.  I tried a few other videos I'd finish edited in the same format. They all do this! Very strange. Again, this is not a problem for me as they are easily fixed, but I'm quite courious about why this is happening! 
Any ideas?  Thanks!

Answer 1

I agree about not bringing a finished video back into Premiere for editing, but I thought I was finished with it and it was not an important project, just an experiment. So when I thought I was finished with it, I deleted the original media to save disk space. And while, as I said, I agree with you on this, that doesn't explain the stated problem.
I also agree that it is probably some mismatch between Sequence Settings and the Export settings. I think this may be because I'm still fairly new to Premiere, and I don't fully understand Sequence Settings.  One window that opens when I first start Premiere is called the New Sequence window. This appears to be where I'd set up Premiere from the specs of my clips (almost always .mov HD files at 1290x720 29.97fps NTSC), but none of the choices in that window equal my footage. Most of them are for DV footage (which indeed has smaller size then my 720p clips).  I suppose I could make a custom setting but I'm not sure just how.
Any ideas?