The singer explained that his and John Lennon's songs dominated the majority of Beatles album, which often left little room for Harrison and Ringo Starr's contributions.
McCartney said the result was that Harrison likened his first solo album All Things Must Pass to "diarrhoea".
"I remember him talking about All Things Must Pass as diarrhoea. That was his own affectionate way of describing that he'd had a lot of stuff stored up and it had to come out. I mean, I don't think I'd describe it like that. But I know what he meant," McCartney told Mojo magazine.
McCartney went on to admit that it upset Harrison that he wasn't allowed to make an equal contribution to The Beatles' albums.
"You know, you can't have everything. It was the Beatles' career and for each of us to have been in the Beatles was pretty amazing and pretty cool," the 'Band On The Run' musician added.
"If it didn't work out how each individual would've wanted it to, then it's... [pauses] ...it's just too bad really because what happened was so good. I think what George did within the Beatles was phenomenal, so I think you kinda have to leave it there."
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Watch George Harrison perform 'All Things Must Pass' below: