This is part of the G1 command line parameter changes enumerated in 6928059.
Currently, G1 will auto-tune the young generation size if the G1YoungGenSize parameter is not set. If G1YoungGenSize is set, G1 will use that as the young gen size.
To be consistent with the other GCs, G1 should observe UseAdaptiveSizePolicy to decide whether to auto-tune the young generation size. NewSize / MaxNewSize (and also -Xmn) should dictate the initial max size.