Hi Kendall:
I used Aperture for years but now use Lightroom. When you choose "Export" in Aperture (and I'm assuming you are exporting a version, and not original), check your export preset and make sure you have the one you want. Also, at the bottom of the drop down menu for the "Export Present" is "Edit". You can always click on this option and it will bring up the settings for the export so you can see what's goin on.
It may be just as simple as an incorrect preset.
Okay....let me revisit this. I JUST tried to run an export on an image and it didn't matter WHAT preset I used or how many dpi or whatever and...I got a small image (501 KB). Then I tried exporting other images and they were fine. Then I went back to try the first image again and it worked fine. That makes no sense. I guess I would say don't give up, keep trying. Apple no longer supports Aperture, so with every iteration of the Mac operating system, it could be there will be glitches. I have the latest version 10.13.3 on my computer.
Good luck!