Environmental Sealings For a camera that you carry everywhere with you, being weather proofed is a big advantage. You will have one less thing to worry about.
Large Full frame (35.6 x 23.8 mm) sensor Large sensor in Sony A9 ensures a good low light image quality compared to smaller sensor, and it also provide shallow depth of field and a nice blurry background when needed.
Medium size Body With relatively smaller body compared to DSLRs and better ergonomics compared to Compacts,Sony A9is a good compromise between size and functionality.
673g Carrying this weight all day may not be very comfortable for you.
Body Thickness 63mm With a thickness of 63mm,Sony A9 is not a pocketable camera for sure.