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 Panasonic FH25 and Nikon D3100 side-by-side from the front, back and top in their relative dimensions. Panasonic FH25 has external dimensions of 99 x 57 x 28 mm (3.9 x 2.24 x 1.1″) and weighs 159 g (0.35 lb / 5.61 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 Panasonic FH25 and the Nikon D3100. Panasonic FH25 is clearly the smaller of the two cameras. Its body is 25mm narrower, 39mm shorter and 47mm thinner than Nikon D3100. As you can tell, actually this is not a fair size comparison since Panasonic FH25 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 Panasonic FH25 and Nikon D3100.
Now lets look at the top view comparison of Panasonic FH25 and Nikon D3100.
Weight is another important factor, especially when deciding on a camera that you want to carry with you all day.
Panasonic FH25 is significantly lighter (346g ) 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 Panasonic Lumix DMC-FH25 (Lumix DMC-FS35) 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
Nikon D3100's 3.00" LCD screen is slightly larger than Panasonic FH25's 2.7" screen.
Unfortunately, both cameras has fixed screens so they don't tilt or flip in directions.