Does Xcode implicitly convert plists in a project to binary during the building process? I don't think it does this implicitly but having a hard time determining for sure.
There is a build setting named "Property List Output Encoding" (a.k.a PLIST_FILE_OUTPUT_FORMAT) which can be used to tell Xcode to convert copied plists to a different format when performing a build. The default value is "same-as-input" though, so it shouldn't do any conversion unless you specifically change that value to something else. If you would like to have your plists converted to binary, to save space and/or load time, just change that setting to "binary" in your target's build settings in Xcode.