To be fair, Jerboa barely has any assets. Basically just an SVG and a few WebPs for the icon and that's it.
Those can increase the size of the app really fast. Think something like just a nice welcome screen with like 5 pages with images that shows you how to use the basics of the app: right there already you need to include those in a few different resolutions for different form factors. Probably at least a few MBs just for that.
It does help to not include proprietary bloated libraries, but code size is not what generally makes the average app big, it's the assets. Liftoff is 10MB, Thunder is 26MB, they both have a bit more visual stuff than Jerboa.