The ~7kb refers to the engine standalone minified and gzipped without any polyfills for browser compatibility (IE11 and friends).
For example on our doc site on https://lwc.dev, you can open dev tools and see the raw engine URL (in UMD format): https://lwc.dev/assets/js/lwc/engine_v100.js (this one is bigger because it contains the synthetic shadow DOM polyfills.
Not sure if lwc-create-app has a PROD mode where it does the minification, but if not you should file a bug.
If you are using tools like webpack or rollupjs, normally these tools bundle in chunks depending on internal heuristics, which is normally a fine approach, but depending on the use case, sometimes you just want to separate the engine by itself to make the cache-ability better.
We will be open sourcing soon a bunch of sample apps and projects so you can see some examples on how to do it. We will add the links into the communities section on the LWC official docs (lwc.dev)
All that being said we will check the number with the latest version and triple-check the size is accurate.