The differences in PP - Physical Partition - sizes is caused by a number of defaults or constants re: the meta data for managing the volumes (aka disks) in a volume group.

The default size for a PP is actually 4M - dating back to when disks were smaller than 4G - because the default number of PP per volume has remained the same - 1016.

When disks were 4G and smaller the VGDA (Volume Group Descriptor Area) was 32M Byte large (or 8 x 4M, so a volume was 32M for VGDA (i.e. 8PP) with upto 1016 (1024 - 8) PP for volume group data.

The VGDA is an important structure - there are three kinds or sizes: normal, big and scaled. Normal is the original and it has space for 32 volumes of 1016 PP each, the big for 128 volumes of 1016 PP each, and the scaled originally began as 1024 disks of 1016 PP each, but now it is meant to scale/grow the max number of PP as the volume grows. But still the original factor of PP per disk is 1016.

Early on (with 4.5 and 9.1 G disks) AIX made changes to affect the number of PP per disk, e.g., so the PP size could be held constant even though disk sizes changed - however, the total number of PP per volume group (a VGDA constant) was fixed. The effect is that changing the number of PP per disk (the -t option of mkvg) means fewer disks can be included in a volume group (normal, and big are fixed at 32 and 128 by defaults).

So, basically, to keep a constant PP size over multiple disk sizes you need to divide the disk size by that PP size to see how many PP that will need, divide that number by 1016, round that number up to the next whole number (i.e., 2033 will need a factor of 3, or 3048 PP (-t 2 if I recall correctly) per volume while 2031 or 2032 will work with a factor of -t 1

Code:

-t 0 - default 1016
-t 1 - 2032 
-t 2 - 3048 
-t 3 - 4064
-t 4 - 5080
etc

Note: adding a disk larger than the original will mean it will not be fully utilized, however, you may change the scaling to a higher number dynamically (say you have been using 75G as a default, and want to add 150G disk/LUN. With no change, only 75G of the disk would be used. Change the scaling: the max number of disks declines, but the 150G disk can be fully utilized.

IMPOORTANT: you cannot change the PP size once a volume group is created. To change the PP size you must backup/restore to a new volume group.

Hope this helps!