While the opcode for both is FF/6, they have the same code segment descriptor, no prefixes were used and the default operand size attribute is 8 or 32 bits?!!
from the manual:
In 64-bit mode, the instructions default operation size is 64 bits. In a push, the
64-bit RSP register (stack pointer) is decremented by 8. A 66H override causes
16-bit operation. Note that pushing a 16-bit operand can result in the stack pointer
misaligned to 8-byte boundary.