Small model and large model are the names of memory models. The small model is for products with a program memory ROM size of 64 Kbytes or less, and the large model is for products with a program memory ROM size of more than 64 Kbytes. In the large model, the number of registers saved on the stack by interrupts, PUSH / POP instructions, etc. increases, so the execution cycle increases. For details on the memory model, refer to "nX-U16 / 100 Core Instruction Manual" P1-20 1.3.6 Memory Model.