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 Canon G1 X III and Canon 5D MIII side-by-side from the front, back and top in their relative dimensions. Canon G1 X III has external dimensions of 115 x 78 x 51 mm (4.53 x 3.07 x 2.01″) and weighs 399 g (0.88 lb / 14.07 oz) (including batteries). Canon 5D MIII has external dimensions of 152 x 116 x 76 mm (5.98 x 4.57 x 2.99″) and weighs 950 g (2.09 lb / 33.51 oz) (including batteries).
Below you can see the front view size comparison of Canon G1 X III and Canon 5D MIII. Canon G1 X III is clearly the smaller of the two cameras. Its body is 37mm narrower, 38mm shorter and 25mm thinner than Canon 5D MIII. As you can tell, actually this is not a fair size comparison since Canon G1 X III has a built-in lens but Canon 5D MIII doesn't. Once you put a lens on 5D MIII, its thickness will increase significantly.
Here is the back view size comparison of Canon G1 X III and Canon 5D MIII.
Now lets look at the top view comparison of Canon G1 X III and Canon 5D MIII.
Weight is another important factor especially when deciding on a camera that you want to carry with you all day.
Canon G1 X III is significantly lighter (551g ) than the Canon 5D MIII 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 5D MIII, the total kit will weigh much higher.
Both G1 X III and 5D MIII have weather sealings in their body, making them resistant to water and dust.
LCD Screen Size and Features
Canon 5D MIII's 3.2" LCD screen is slightly larger than Canon G1 X III's 3" screen.
On the other hand, Canon G1 X III 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. On the other hand, Canon 5D MIII has a fixed type screen which provides almost no flexibility in shooting positions compared to G1 X III.