TEMP0+TSRAGR,
Your line of thinking is correct regarding the IAE engine at altitude.
The CFM 56-5A1 has a fan diameter of 1.830 m, bypass ratio of 6, and pressure ratio of 25.6.
The IAE V2500-A1 has a fan diameter of 1.681 m, bypass ratio of 5.4, and pressure ratio of 29.4.
At low altitude where the air is thick the fan is producing most of the performance for the aircraft, up high with thin air, the engine core is producing most of the performance. As the IAE engine has a lower bypass ratio and higher pressure ratio it is “closer” to a turbojet design (more pure jet thrust) hence the better performance at altitude.
Regards
Z