Files in zip archives can be compressed, but don't have to be. If you compress resource.asrc, there are bad runtime consequences; much worse than the package bloat that results from not compressing it.
Are you saying that the SDK tools that build apks (aapt? I'm not sure) only compress files selectively? And what kinds of consequences are there if you do compress resources.arsc? I suppose it never keeps a copy in memory but reads it a lot, and if it's compressed, you're taking a performance penalty from it having to decompress the whole thing on every access?
Yes. Take a random apk and look at it with a zip lister. If resource.asrc is compressed, someone didn't use the SDK tools to build it (and I'd be shocked).
What you described is about my understanding of what happens. I would hope it doesn't uncompress the whole thing on every access, but only far enough to read the value; but I'm not sure. Note that it's not just the app itself that uses the app's resources; other elements of the OS use them too.
