For example, I've seen people allocating 4GB buffers for everything, just because that made it impossible to have buffer overflows as long as they indexed with 32-bit integers. But on x86-64 you can only fit 65k such buffers in the address space, and it's not that hard to exhaust that.
Still, even if you design for computers with 256GB (2^38) physical memory, you still have a virtual address space that's 2^9 times larger (assuming addresses with the MSB bit set are reserved for the kernel's memory space). This is opposed to 32 bit systems where the physical memory space is close to or larger than the virtual address space. E.g. high end smartphones sold in the last few years have 2GB RAM and only 3GB of virtual address space.