Let's break down the differences (SR4 to SR4A to SR5):
Flavor Text/stories:
SR4: 8+(1 x 12) = 20 pages
SR4A: 8+4+4+4+4+4+4+4+4+4+4+4=52 pages
SR5: 6+4+4+4+4+4+4+4=34 pages
History:
SR4: 12
SR4A: 14
SR5: 4
6th world life:
SR4: 15
SR4A: 15
SR5: 19
Game Concepts:
SR4: 18
SR4A: 15
SR5: 13
Creation:
SR4: 18
SR4A: 14
SR5: 48
Sample Characters:
SR4: 16
SR4A: 15
SR5: 15
Skills:
SR4: 26
SR4A: 22
SR5: 25
Combat:
SR4: 31
SR4A: 27
SR5: 51
Matrix:
SR4: 35
SR4A: 32
SR5: 45 +7 (rigger-only chapter) = 52
Magic:
SR4: 41
SR4A: 36
SR5: 51
Note: the next 3 chapters are heavily rearranged from 4th to 5th, so their raw length as listed here does not mean much. However, there is a LOT more to the GM Advice chapter, including drop-in locations and so on, in addition to incorporating some of the elements from other chapters.
Friends & Foes:
SR4: 27
SR4A: 26
SR5: 37 (includes most parts from RtS, below)
Running the Shadows (minus GM Advice):
SR4: 23
SR4A: 18
SR5: (included in above)
GM Advice:
SR4: 5
SR4A: 6
SR5: 44
Gear:
SR4: 44
SR4A: 41
SR5: 51
So where did the extra book space go? Pretty evenly spaced between character creation, magic, tech, combat, GM advice, and gear.