I tried to build the image myself, ends up with 689MB as an image. I removed the entrypoint from the Dockerfile (this line), so I can run the image interactively (docker run -it <image_name> sh).
Then I poked around the filesystem. Turns out that the /opt folder is the biggest so far, here's the content:
If you take a look in that folder, you'll find that the .local folder contains the JAVA jre which is needed to run Mendix, in the runtimes folder you'll find the Mendix runtime, also needed.
Combine that with the fact that mendix/rootfs is about 200Mb, this explains the size of the image.
That being said, I'm curious how a somewhat larger project suddenly takes 400Mb more space. Could be worth looking around in the filesystem to see where everything ends up.
For reference, I followed some of the ideas to inspect and shrink Docker images from this Medium post.