In Geoff Emerick's book (I can't give a page reference as I'm reading the i-book version) it has the following passage in the Abbey Road chapter ...
But by the time I arrived, an agitated John was deeply involved indeed. More specifically, he was having a row with Paul and George Martin.
'We've already done the concept album,' he argued, presumably referring to Pepper. 'Why do we need to do another one?'
'Look John, we're just trying to think symphonically,' George replied. 'We're trying to create a complete work out of song fragments.'
John was derisive at first, saying, 'You're taking yourselves too seriously,' but when Paul invited him to contribute some compositions of his own to the medley, he seemed to capitulate. 'Well I might have one or two that could fit,' he said sheepishly.
Some proof (if Emerick can be believed) that it was Paul's idea, with the support of George Martin, to create a "symphonic" piece by joining up some shorter song fragments.