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 (35.9 x 23.9 mm) sensor
Large sensor in Sigma fp 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,Sigma fpis a good compromise between size and functionality.
Carrying this weight all day may not be very comfortable for you. Body Thickness 45mm
With a thickness of 45mm,Sigma fp is not a pocketable camera for sure.