Can MCU ROM and RAM limitations be scaled?
A pure micro-controller has the memory busses (the wires and logic that connect the CPU to its ROM, RAM and peripherals) internally, hence there are no wires to connect an external ROM or RAM to. You are stuck with whatever ROM and RAM the manufacturer has included in the chip version that you are using. In most cases the manufacturer offers a range of related chips, the ROM and RAM size being one of the distingctive features of the various chips. You choose the chip that fits your needs. Your SAM3X8E seems to be in this category.

A pure micro-processor has no internal memory, and hence has external pins that are used to connect external ROM and RAM to. You choose the ROM and RAM to meet your needs.

There are hybrid chips, that have both internal RAM and ROM and pins that can be used to connect external RAM and ROM. (In some cases external memory can only be used for code, or only for data.) Such chips can be used either in microcontroller mode (without extrenal memory) or in microprocessor mode (with external memory).