Probably video RAM
The system ram in each system is same, but the VRAM in PS2 was low volume but very, very fast speed EDRAM (which is why the games were still very pretty, much more sophisticated effects than even PC was doing at the time). However PC ports would have low texture quality because of this too.
The PS2 EDRAM was doing 48 GB/s. In comparison today's GPUs say on the PS4 Pro is about 200 GB/s
So difference of 13 years between PS1 and PS4 and the difference is only about 3 to 4 times in terms of JUST GPU bandwidth, which is why PS2 is probably more difficult to emulate (though shouldn't be so hard just based on that raw data since GPUs on PC well dwarf this as well) and why it was difficult to port games for.
The DC had 8 MB VRAM iirc and the PS2 had 4 MB of the EDRAM.
And as comparison NV2A in Xbox had 64 MB of RAM which I think was system RAM shared between gpu and cpu also