why is this book so big?
The ctg file is massive (4Gb). This is a huge size for a ctg book. Even reference type openings books that are meant to cover all possible openings and not for engine play are never this big and don't need to be. I compared Poisoned Arrows 2 and Poisoned Arrows 3 to see why such a big increase. PA 2 was made from 110,492 games whereas PA3 has been made from 678,960 games. So basically the author (or someone else) has taken PA2 and added about half a million games. As the book goes about 42 moves deep, this huge number of added games would explain the massive file size.
I have had a quick through and the hand tuning/move colouring/move weightings of PA3 look very similiar to PA2 so I don't think there has been much extra tuning done here - mainly taken PA2 and added in a shed load of games which has changed the statistics slightly but not much. Doing this actually does not add anything to the strength or performance book cos it has been tuned to play only a very limited number of lines (as white it will only play 1.e4 and in response as black is tuned to respond along B90 line). so as its only playing a narrow range of lines then it doesn't matter if it has been made from 100 thousand or 100 million games - it will still play the same moves in the same lines.