iTunes music file size marginally different than iPhone music file size

When using Apple Music or iTunes Match to sync your library and playlists automatically your device then the device will generally get 256k AAC matched copies of your media. Even when iTunes uploads your tracks because they cannot be match it will likely convert higher quality tracks to 256k AAC. Short of switching to direct syncing with iTunes there is no way to get the AIFF versions to copy to your device.

tt2

Pretty much yes. The song in your primary library should remain unchanged (unless you remove the download, then redownload from the cloud) but what gets provided to other libraries and devices will normally be in 256k AAC format. This will usually be a matched copy from the iTunes Store, or a file uploaded from your original library, converted if needed so that it isn't excessively large in comparison to the default format. I.e. you may get an uploaded 256k MP3 back unaltered, but WAV, AIFF, and ALAC are always going to get converted.

tt2