Just got another email from them this morning, they've figured out what's going on.
It turns out they had a bit of a mix up, and the problem actually isn't fixed in the most recent public build, but will be in the next one.
Here's the email they sent me:
Thank you for taking the time to help us figure out this issue. I must admit there was an mix up on our end. I had thought the fix for your issue was in version 18.1 which was released on March 27th. It looks like the fix was not checked in until March 30th. This of course means the fix was not in version 18.1.
For now there is nothing more we can do to shirk MotionX GPS down to size. However, look for version 18.2 in the iTunes app store within the next month. If the issue is not resolved after opening and closing version 18.2 a few times please let me know. It should only take about 10 times of opening and closing the app before you are at the correct app size.
So, we just need to wait until the next release... and then every time we open the app the wasted space will get a bit smaller until it's gone.