X31 Cannot get over 1024x768 on external monitor @32 bpp

[virtualsim]

I've been battling with this for some time.

On my X31, whenever I try to get an extended desktop using a resolution greater than 1024x768 on the external monitor, I cannot use true colors.

If I use 16bit colors, it works as it should, but when I try using 32 bits the external display reverts to 1024 although it displays a virtual space corresponding to the selected resolution (going to the edges of the screen will scroll the background).

Would 16 Megs of video RAM not be sufficient to display both desktops ?

My quick calculation of memory needed for a 1024x768 desktop @ 32bits comes to 3072 KB.
a 1280x1024 32bpp desktop would require 5120KB.

Using both displays would come to a total of exactly 8MB.

Why is it this cannot be displayed on a machine with 16MB ?