Some windows 32 bit versions cannot allocate more than a gigabyte to your server, quite common. You could try 1024 mb and 2048 mb digits instead of 1-2 gigas.
I hope it helpd
Due to the limited space available with a 32-bit address bus all 32-bit windows only allow a total of 3GB ram total, and most older versions required 1-1.5 GB restricted for exclusive Windows use. If you have "Integrated" graphics (no video card) then another 256-512 ram is taken from the total, leaving around 1GB for Java/Minecraft.
You can technically tweak the Windows config for a total of 4GB but you really have to know what you're doing and hope your hardware allows the extra ram to be installed.
64-bit Windows does not have this small limitation because of the 64-bit address bus. Here are the comparative ranges:
Size: Locations:
32-bit: 4,294,967,296
64-bit: 18,446,744,073,709,551,616