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 L31 and Nikon D5000 side-by-side from the front, back and top in their relative dimensions. Nikon L31 has external dimensions of 96 x 59 x 29 mm (3.78 x 2.32 x 1.14″) and weighs 160 g (0.35 lb / 5.64 oz) (including batteries). Nikon D5000 has external dimensions of 127 x 104 x 80 mm (5 x 4.09 x 3.15″) and weighs 590 g (1.30 lb / 20.81 oz) (including batteries).
Below you can see the front view size comparison of Nikon L31 and Nikon D5000. Nikon L31 is clearly the smaller of the two cameras. Its body is 31mm narrower, 45mm shorter and 51mm thinner than Nikon D5000. As you can tell, actually this is not a fair size comparison since Nikon L31 has a built-in lens but Nikon D5000 doesn't. Once you put a lens on D5000, its thickness will increase significantly.
Weight is another important factor especially when deciding on a camera that you want to carry with you all day.
Nikon L31 is significantly lighter (430g ) than the Nikon D5000 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 D5000, the total kit will weigh much higher.
Unfortunately neither Nikon Coolpix L31 nor Nikon D5000 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 Compact Cameras with Weather Sealing Top DSLR Cameras with Weather Sealing
LCD Screen Size and Features
Nikon L31 and Nikon D5000's LCD screens has the same diagonal size of 2.7".
On the other hand, Nikon D5000 sports a Fully Articulated screen
which you can change the angle and position of the screen almost freely, making it possible to shoot from waist or over the head levels, and especially very useful for shooting selfies and video. Nikon L31's LCD screen is a fixed type which provides almost no flexibility in shooting positions compared to D5000.