If you recall the In His Own Words interview entitled "What Happened", he said "Listen to 'Party and Bullshit'. Listen to his style. Changed from that to 'Big Poppa'... Biggie had just took my shit."
Basically how Pac felt was that Biggie had a shoot-em-up style (evident in "Party and Bullshit" and "Dead Wrong" as well, which was recorded before Ready to Die) and switched it to the style that Pac had, which encapsulated the thug, the player, the man coming to grips with the possibility of death, etc. perhaps.