uhm, actually plist files are xml
ACTUALLY plist files can be either textual or binary, which is very much not XML
I should have said not necessarily though, instead of just "not"... but it was kind of irrelevant to the main point.
They certainly aren't very compact as far as formats go, even on the watch.
Sigh, didn't read much of that original message, did you?
They don't NEED TO BE EXTREMELY COMPACT because they are sent over only once, when the app is loaded on the watch - that said, it is in the binary format which is much more compact than the textual format.
In use the watch pulls files from that bundle at runtime. And if you were any kind of programmer you'd know there is a tradeoff between compression and computation (which the watch has little of) in terms of file formats, so a fairly but not maximally compact file format is better for performance than whatever you are thinking of.