If you are running AIX 6.1 or above you should have set the "edition" type when you installed it. The default seems to be "Express" which restricts the size of memory and number of procs in a partition.
There are three editions for AIX 6.1 and 7.1:
Express which is the basic/entry level and was designed to make available a low-cost version of the OS, and is generally constrained to a maximum of 4 cores and 8 GB of memory per core in a single partition, and there are also limits on cluster-aware-AIX in AIX 7.1.
Standard edition is the one IBM expects most people to use, and basically is only limited by the hardware IBM offers you.
Enterprise which includes many extra tools for clustering and monitoring.
To check which version you are running:
# chedition -l
To switch editions:
chedition: Change to express edition
chedition -x [-d Device [-p]]
chedition: Change to standard edition
chedition -s [-d Device [-p]]
chedition: Change to enterprise edition
chedition -e [-d Device [-p]]
The command can be run with a reboot, however it is unclear as to how this affects AIX's actual usage of the system's resources. IBM has also recently announced that a lot of the restriction imposed by the Express edition will now only apply to compute-nodes in their Pure systems.