I've read the max heap size on 32bit Windows is ~1.5GB which is due to the fact that the JVM requires contiguous memory. Can someone explain the concept of "contiguous memory" and why you only have max 1.5GB on Windows?
Secondly, what then is the max heap size on 64 bit Windows and why is this different than what's available on 32 bit?
The 32-bit/64-bit part is unrelated to Java
It turns out that memory locations in a 32-bit system are referenced by 32-bit unsigned integers. This allows up to 2^32 possible memory locations. Since each location stores 1 byte you get 2^32 bytes or 4 GB if you prefer.
On a 64 bit system there are 2^64 locations, or 16 exabytes.
Now, in Windows, the contiguous part becomes a big issue, but that is just how Windows does things. The idea is that you need to have an entire "uninterrupted" range for your heap. Sadly, Windows allocates some memory somewhere in the middle. This basically leaves you with about half the left side or half the right side, about 1.5-2GB chunks, to allocate your heap.
Check out this question for more details on 32 vs 64 bit.
Edit: Thanks mrjoltcola for the exa prefix!
Contiguous simply means "without gaps", one long single segment. The amount is limited by how large a segment the OS can map for your process. Whether Java requires a contiguous heap or not is an implementation issue specific to JVM and may not exist for other VMs.
Contiguous memory is not the problem limiting windows to use only 1.2 GB of heap. Even though min/max heap defined, JVM would occupy max heap from system memory while starting. It will then reference only min heap within the occupied system memory until it had to expand. Contiguous memory of max heap is required to start JVM in most of the implimentation to improve performace.
As Marcus explained above the limit of 32 bit hardware is 4GB for a single process(thread). Every operating system address this 4GB diffrenetly. 4GB is majorly split as kernel space and user space. In 32 bit windows the max user space is close to 1.5 GB. There is option to boot windows with /3GB switch to have more userspace.