Using the WPTAgent (version as of 1wk ago) and WPT 17.12, I have two Apple iOS11 devices running in a private instance and the bytes-in (downloaded) size is correct for some objects served with GZIP and wrong (=uncompressed size) for other objects also served with GZIP.
The issue is repeatable over multiple tests and the different iOS devices and always seems to affect the same objects in each run. Several objects in each run report the incorrect bytes-in size.
The issue has also been replicated on public WPT using iPhone6 at: https://www.webpagetest.org/result/180503_BC_68f059dfa4526a36d47a91e514dcf053/1/details/#waterfall_view_step1
e.g. for two objects from the same domain and each with a Content-Encoding: gzip response header:
object #12 is shown correctly with bytes-in < uncompressed size
object #15 is shown incorrectly with bytes-in = uncompressed size.
Same site tested on desktop, Android via WPT, emulation in Google Chrome and HTTPWatch on an Apple device all show the content is correctly compressed.