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I'd say that's a fairly conservative estimate. From a quick look through Anytime plus on my box, a standard-def film is about 1gb, high def can be 3gb or over, one of the HD films I looked at was nearly 3.7gb!
It can be a bit weird, I was trying to get the 2nd episode of Game of Thrones the other night, it initially said the HD version (51 minutes) was about 2.3GB, however having downloaded 1% it was then saying it was 11.5GB! As that would have taken me well over my monthly d'l limit I cancelled it and got the SD version, which was 680MB.