Size and weight is a big decision factor when you are trying to find the ideal camera for your needs.
In this section, We are going to illustrate Nikon D80 and Nikon S3100 side-by-side from the front, back and top in their relative dimensions. Nikon D80 has external dimensions of 132 x 103 x 77 mm (5.2 x 4.06 x 3.03″) and weighs 668 g (1.47 lb / 23.56 oz) (including batteries). Nikon S3100 has external dimensions of 94 x 58 x 18 mm (3.7 x 2.28 x 0.71″) and weighs 118 g (0.26 lb / 4.16 oz) (including batteries).
Below you can see the front view size comparison of Nikon D80 and Nikon S3100. Nikon S3100 is clearly the smaller of the two cameras. Its body is 38mm narrower, 45mm shorter and 59mm thinner than Nikon D80. As you can tell, actually this is not a fair size comparison since Nikon S3100 has a built-in lens but Nikon D80 doesn't. Once you put a lens on D80, its thickness will increase significantly.
Comparison image of Nikon D80 and Nikon S3100 Size, Weight and External Dimensions - Front View
Now lets look at the top view comparison of Nikon D80 and Nikon S3100.
Top View Comparison image of Nikon D80 and Nikon S3100 Size
Weight is another important factor especially when deciding on a camera that you want to carry with you all day.
Nikon S3100 is significantly lighter (550g ) than the Nikon D80 which may become a big advantage especially on long walking trips.
Again, this comparison may not be very useful in real life as we are comparing the weight of a camera with a built-in lens to the body-only weight of an interchangeable lens camera. Once you put a lens on D80, the total kit will weigh much higher.
Weather Sealing
Unfortunately neither Nikon D80 nor Nikon Coolpix S3100 provides any type of weather sealing in their body, so you have to give extra care especially when you are shooting outdoors. If weather sealing is a must-have feature for you, check these sections: Top DSLR Cameras with Weather Sealing Top Compact Cameras with Weather Sealing
LCD Screen Size and Features
Nikon S3100's 2.70" LCD screen is slightly larger than Nikon D80's 2.5" screen.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.