Strange increase in size

It is quite simply bit rate.

720p video is normally encoded at around 10Mb per second.

Your original 69 minute video was encoded at a ridiculously low bit rate, probably around 1Mbps which is why the new video is nearly 10 times bigger.

You can reduce the bit rate when you export by selecting Custom in the Quality section and dragging the slider all the way to the left.

With a 720p project you can drag the bit rate down to 2Mbps which will probably still result in a video a bit larger than the original.

Likely the video that you imported into iMovie was more highly compressed than the video that you exported out, which can account for the larger file size. When a clip is imported into iMovie, iMovie unpacks it for editing. If the imported clip were highly compressed, then when exported out iMovie's compression would likely be much less. Thus larger file size. Less compression usually is a good thing. However, 2.29 GB for a 69 minute video is almost absurdly low. I imagine that the quality is pretty poor.

Other things that contribute to file size are compression settings, resolution, and bit rate.

You can try sharing out to email where you will have small, medium, and large options under the resolution settings, and then dragging the movie out of the email to your desktop.

-- Rich