"The X Factor" -- Half the Show Of "American Idol"

The big "X Factor" premiere last night fell way short of Simon Cowell's expectations -- earning just over half the ratings "American Idol" got during its premiere in January.

Simon's new show averaged an 8.7 rating, while the tenth season premiere of "Idol" pulled in a 16.4 -- and in last night's head-to-head ... "Modern Family" and "Criminal Minds" won out.

Simon called in to TMZ Live yesterday -- telling us, he desperately wanted to beat out his alma mater in the ratings department ... but based on last night's performance, that's gonna be an uphill battle.

To be fair -- the first ever episode of "American Idol" only scored a 6.1 in 2002 ... and has since had years to build a loyal audience. Then again, singing competitions were new back then.