Dave Grohl -- Foo Fighters Emmy Diss ... You Can't Cut Our Songs Short

Now we know why Dave Grohl was so pissed about the Foo Fighters getting yanked from the Emmy Awards stage -- and it's all about size. Yes, it matters ... especially at award shows.

A rep for Grohl tells TMZ ... the band was all set to perform, and would have been the first ever rock band to play the Emmys. But the rep says, "FOX then refused to allow the band to play a full song from the Emmy-winning Sonic Highways."

According to our sources, the network wanted the band to play half of their winning song and then play part of their hit song "Learn to Fly" ... but the band wasn't having it.

We're told the directive came straight from the network and not the television academy, who was on board with the band's plan to play what they wanted to.

FF won 2 Emmys for its HBO docuseries, but you can tell from his reaction last night ... Dave wanted to play, but obviously ... he wanted it his way.

We have a call in to FOX and the Emmys for their side of the story.