It's likely not the size of the RAM, but by while upgrading the memory you were likely also widening the datapath of the RAM to the VGA controller (probably from 16-bit to 32-bit).
Many 32-bit VGA controllers allowed the RAM to operate in 16-bit mode as a cost cutting measure.

Depends also on your exact cirrus chip. Some later card used memory interleaving. Recently i read about some PCI models, which had it enabled after increasing memory from 1MB to 2MB.

Ah so something does change on the card! Thank you guys, that was enlightening.

konc wrote on 2020-11-24, 12:52:

Ah so something does change on the card! Thank you guys, that was enlightening.

In this case: yes. But be careful with generalizations.

There are a lot of RAM upgrades on old cards that really do almost nothing; famously the CL-GD5428 can be pushed up to 2MB, but all it gives you over 1MB is a few eye-melting interlaced modes. No extra colours in non-interlaced modes, no extra performance. Check the datasheets of the VGA controllers to be sure: if you don't see any reference to what extra RAM does that's a pretty good indication it doesn't do much. For performance the thing to watch are bus width and interleaving.

I have a newer example of this, a CL5434 PCI that enables 64 bit bus when upgrading fron 1mb to 2mb.
https://en.wikipedia.org/wiki/Cirrus_Logic
And also noticed an important performance increase when upgraded a S3 805 VLB like this one https://vgamuseum.ru/gpu/s3-graphics/s3-805/

Just curious. Do you get anything with ET4000/W32P vlb card if you have 2MB instead of 1MB?

Only when accelerated graphics is in use (in Windows).