ANSI and Chinese SS7 point codes are probably bigger as they need to address more network elements in their geography than other national networks.

Japanese point codes are different, just because. [I'd like to know if there is a reason]

For the record, ANSI point codes are often written in 8-8-8 format (network, cluster, member) whereas ITU pointcodes are more commonly represented in decimal or hexadecimal. However they can be written in 3-8-3 format or any local convention (up to and including 7-7).

ANSI point codes are allocated by Telecordia, ITU national point codes are allocated by a national authority.