The MMCX (Micro-Miniature Coaxial) connector series is a micro-miniature version of the MCX. It has the all features of the MCX series, including 50 Ω impedance with a broadband capability of DC to 6 GHz and secure, fast and easy snap-on/ snap-off coupling, and a wide range of available connectors. However, MMCX connectors have a smaller foot print than the MCX series.
Here are examples of both - The MCX 
And the MMCX 
You can verify also by the part drawings to see dimensional differences
MCX drawing http://www.amphenolrf.com/media/downloads/3575/252180.pdf
MMCX drawing https://belfuse.com/resources/Johnson/drawings/dr-1353403001.pdf