Trimming video triples the file size

I start with a 100 minute MP4 video (1920x1080-30) that is 2 gb in size. I can trim the original by trimming the beginning and the end and have a 40 minute video that is 6.5 gb in size.
1) Am I doing something wrong?
2) How can I end up with a video of the same file size or smaller?

Sounds like your are using 'automatic/best' (the default) for kbps on export.
Try changing the Kb/s and restrict it to the rate of the original, or something other than auto as it can crank up the Kb/s and cause large file sizes.

kbps.jpg (52.95 KiB) Viewed 752 times

If trimming is all you usually do using a lossless cutter might be more appropriate.

TMPGEnc Smart Render
Machete
Lossless Cut
Video Redo
Etc

Your original file has only about 2.6Mbit average bitrate, so you have to keep it the same to get relatively same file size.

If you want to just trim file or do hard cuts the best solution is one of the mentioned tools, which will keep original data.

restrict it to the rate of the original

Where do I find what the rate of the original is?

MediaInfo is free. But you may not like the quality at the same bitrate.
Why don't you try the Media Manager with the Trim option? It'll leave your encoding untouched, if it can handle the codec.