Do any iOS 9 users notice some apps having smaller sizes now? I know Apple fixed the feature, but don't know if it is being properly implemented.
It's hard to tell. App thinning consist of 3 components, slicing, bitcode, on demand resources.
As far as slicing goes it still appears the apps on my iPhone contain all the unused iPad elements.
None of the apps I use would benefit from on demand resources. These would need to be apps that intentionally won't use the entire app until later on. Apple gives the example of a later levels of a game that you won't get to until you beat the earlier levels.
And as far as I can tell bitcode would be something that is virtually impossible to distinguish from the users end.
Things I can tell, excluding cache app sizes are the same between unlike devices (iPad and iPhone) and having 8gb of app data (not other) on an unused 5S then updating nearly every app still remains about 8gb. So if there was any savings before app thinning and now it's minimal.
That's just my experience. I still find the best method for freeing space is to restore as new so all the app cache is wiped and all the other data is removed.