Smaller pockets mean the jeans can be more form-fitting, and women's clothes are always marketed towards the fashion industry's definition of beauty... Basically why have something functional, us women are just meant for looking pretty :/ (I'm clearly not bitter. No way.)
Seriously, I had no idea this was a thing that was needed until my wife explained why she doesn't keep anything in her pockets. I was shocked to see how tiny they were!
That's my theory at least