It could have 5TB of memory, it wouldnt make a difference. The memory in your system has far greater latency than the memory on a graphics card, and is generally significantly slower, with a smaller bus width as well. IGPs also have fewer processors that tend to be lower performing over all to keep their heat production down.
GPUs work similarly to a CPU, if you had a pentium 2 with 4GB of ram you wouldnt expect it to run fallout 3 well, it has plenty of RAM but it isnt nearly fast enough to need that much, no matter how much memory you gave it it just wouldnt have the processing power to give you decent performance. The same applies to the GPU.