It seems like the more headliners there are, the more they can get away with them being smaller names. In 2017, there were only 3 headliners but they pulled in Eminem and Muse, huge acts. They're trying to cater to too many genres this year (hip hop, grime, rock, indie, dance). I would say only Stormzy and Post Malone are 'huge' acts this year. LG is popular but it's just him on his own and half his set will be Oasis songs played by a backing band of nobodies