There's a lot of reasons but 32bit is only enough to address 4Gb of memory. It works out in the conversion factor but it's kinda difficult to explain. 64bit have a total of about 190ish GB if I remember correctly. And the memory capacity is accross your ram and GPUs. So not only are you overloaded on your RAM, but the system can't see the GPUs memory either. Switch up to 64bit and suddenly you'll lose a giant bottleneck.