Size and weight are big decision factors when you are trying to find the ideal camera for your needs.
In this section, we are going to illustrate the Casio EX-Z450 and Nikon D3100 side-by-side from the front, back and top in their relative dimensions. Casio EX-Z450 has external dimensions of 81 x 56 x 21 mm (3.19 x 2.2 x 0.83″) and weighs 128 g (0.28 lb / 4.52 oz) (including batteries). Nikon D3100 has external dimensions of 124 x 96 x 75 mm (4.88 x 3.78 x 2.95″) and weighs 505 g (1.11 lb / 17.81 oz) (including batteries).
Below you can see the front-view size comparison of the Casio EX-Z450 and the Nikon D3100. Casio EX-Z450 is clearly the smaller of the two cameras. Its body is 43mm narrower, 40mm shorter and 54mm thinner than Nikon D3100. As you can tell, actually this is not a fair size comparison since Casio EX-Z450 has a built-in lens but Nikon D3100 doesn't. Once you put a lens on D3100, its thickness will increase significantly.
Here is the back view size comparison of the Casio EX-Z450 and Nikon D3100.
Weight is another important factor, especially when deciding on a camera that you want to carry with you all day.
Casio EX-Z450 is significantly lighter (377g ) than the Nikon D3100 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 D3100, the total kit will weigh much higher.
Weather Sealing
Unfortunately neither the Casio Exilim EX-Z450 nor Nikon D3100 provides any type of weather sealing in their bodies, 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 Compact Cameras with Weather Sealing Top DSLR Cameras with Weather Sealing
LCD Screen Size and Features
Casio EX-Z450 and Nikon D3100's LCD screens have the same diagonal size of 3.00 inches.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.