Full desktop of a standalone ? Seemingly no option

An interesting assumption was made by one of the users, he said that the maximum resolution in a cube cannot exceed a certain number of pixels, if I understood him correctly.
Here is his post on the forum:

I tried setting the screen resolution inside the Linux HVM cube to 2560x800 and it worked.
I tried adding a screen resolution inside a Linux HVM cube of 2560x900 and it didn’t work, so the person was right, the maximum screen resolution inside a cube cannot exceed a certain number of pixels.
But if this is so, it is not clear why Windows HVM has the maximum screen resolution as in dom0 ?
Perhaps it’s a driver issue ?
I don’t understand…