PS2 had some of the most amazing cost reductions ever.It's just a cost reduction. PS2 and PS3 had like 20 revisions.
https://www.psdevwiki.com/ps3/Motherboard_Revisions
https://playstationdev.wiki/ps2devwiki/index.php?title=Motherboards
It became the size of a DVD case in its FINAL FORM!
The PS3 and PS4 barely got anything that can be called slim. And they were/are sold at considerably high price compared to older generations