Consider the question. On January 19, 2017, it was announced that the SIG Sauer P320 MHS variant had won the United States Military's Modular Handgun System trials. The P320 will be known as the M17 (full size) and M18 (compact) in U.S. Military service. Though the pistol will remain chambered in 9 x 19mm Parabellum rather than a larger caliber, the contract allows the Army and other services to procure Sig Sauer's proposed XM1152 Full Metal Jacket and XM1153 Special Purpose ammunition. The ammunition chosen to go with the pistol is a "Winchester jacketed hollow point" round.

What is the difference between the m17 and m18?
If the answer is 'M17 ( full size )'; explain the reasoning:
To answer this question, we should know that: The P320 will be known as the M17 (full size) and M18 (compact) in U.S. Military service.