Here's the deal:
I imported a .mov file that was 68Mb. I needed to cut out some parts of the video that contained text/titles, so I cut those out and then nested the newly separated clips. This created a sequence. (I actually wanted to create a single clip out of these "merged" clips, but it created a sequence instead. Maybe this is part of my problem?)
In any case, I clicked on the new sequence and exported it using quicktime conversion. I chose H.264 as my format. I kept the frame rate and frame size the same, and I even exported with no sound. And still the exported file was more than twice the size of the original (only slightly modified) file.
Any ideas why this happened? In general, I'm trying to compress, not expand, the files I export.