In practice there are only two things that make up the filesize of Construct 2 games:
1) Audio
2) Images
For 1), just prefer to use a few short sound effects rather than lots of long audio clips. Music is going to take a lot (3-4mb for a single track) so you probably can only fit one track in, or maybe even just a short loop.
For 2), avoid using lots of animation frames. Also see the blog post Kyatric linked to for the image compression options that can help reduce the filesize.