The Brain Size displayed in front of a puzzle pack is not related to the points required to unlock that pack. This brain size shows the total points you will receive for completing all the puzzles within this pack. For example, if you complete all puzzles within the pack ‘ANT’ you will receive 20 points.
The next puzzle pack unlocks when you have successfully completed all the puzzles in the previous pack.
As you successfully solve the puzzle and advance in the game, your score keeps on adding which is depicted as 'Brain Size'. So, your total Brain Size is the cumulative score of all the puzzles you have solved so far in the game.