The Oracle Java Virtual Machine in a Windows 32 bit environment is limited to under 1.5GB. The exact limit, and how close you can push to that limit, will vary depending on what you are doing in Windows. The JVM requires contiguous memory and Windows may not have this much unallocated contiguous memory to spare.

I suspect the 1GB max is an attempt to pick a safe value for 32 bit Windows.

and 640 MB under 32-bit Windows. This value is indicated in the About dialog.

That's what I'm confused about. My About window shows 1 GB Max.

Caution : I think that if you run a 32-bit Windows system on a 64-bit processor, Windows will let programs run in 32-bit mode only.

This is correct. I'm fully aware of that. [;)]
It was the best suited processor available for my motherboard, for my needs at the time, way back when I built this rig about 7 years ago.