Large APS-C (23.1 x 15.4 mm) sensor Large sensor in Nikon D3100 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.
Large Body Even the smallest lens attached to this body will be a burden to carry everywhere with you.
505g Carrying this weight all day may not be very comfortable for you.
Body Thickness 75mm With a thickness of 75mm,Nikon D3100 is not a pocketable camera for sure.