Dr Kermode is known for waving his rather large hands when he pontificates upon the merits of a film. This is usually exacerbated when the pontification becomes a full on Kermodian rant.
This originated when Armando Iannucci noticed how massive Dr Kermode's hand are, and wrote them into the dialogue of The Thick Of It (S03 E05, first broadcast on 21 November 2009), as a throwaway line. This is according to an interview on Richard Bacon's real-life BBC Radio 5 Live programme when Richard was telling Armando how thrilled he was to appear on The Thick Of It. In the episode's story, Malcolm Tucker (Peter Capaldi) before a visit to the BBC Radio 5 Live studios for a fictional in-story interview with Richard Bacon, says Mark Kermode has "huge flappy hands". Later, at the end of the episode Ollie Reeder (Chris Addison) says while looking at Kermode on a TV screen, "God Kermode, your hands are massive!". Guardian review of the episode
This version of events was contradicted by the Good Doctor himself, however, who said that he spoken to Iannucci at the Baftas, and Iannucci apologised for the hands becoming a thing, adding that had said he had given the writers half an hour to work out a gag "and that was what they came up with."
By the time it got to the Paranormal Activity review on 27th November 2009, Simon had been tipped off to mention hand-size. Mark was using his flappy hands to recreate how the audience were scared in the trailer for the film, repeatedly mentioning "tiny things", to which Simon replied in a deadpan voice "You have got big hands, haven't you?". Mark said "Yeah, thank-you(!). You, and The Thick Of It...".