Any images or sounds not added in the Construct 2 editor are not included in the offline cache, because there's no way for the editor to know about them. Therefore any images you request are not guaranteed to be saved anywhere. This means you could play the game 3 times in a row and it re-downloads the images every time, or if you go offline then the game fails to run at all. It's best where possible to keep all resources within the Construct 2 editor itself so they can be properly cached and loaded offline.
The best tips to reduce file size are probably the image-related parts from this guide: https://www.scirra.com/tutorials/577/construct-2s-export-time-optimisations
- duplicate images don't matter, they're deduplicated on export
- spritesheeting can help compress away similar parts of animations
- images with <256 colors will automatically be recompressed to much smaller PNG-8
- use Construct 2's Image Format dialog to manually select JPEG or PNG-8 where appropriate
Selecting JPEG on backgrounds alone can massively reduce the file size.
Don't trust Construct 2's 'estimated download size' too much. Everything that happens on export is so complex that it's impossible to accurately predict what it will result, so for best results from time to time you should actually export the project and see how big it ends up.
Note this is specifically to do with the download size, not memory use. Don't get them confused! Although they tend to roughly correlate, it is not helpful to be looking at memory use if you're actually trying to measure the download size. A0Nasser posted a screenshot of the debugger which only shows memory use and won't tell you anything about the file size.