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 1Ds 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 1Ds MIII has external dimensions of 150 x 160 x 80 mm (5.91 x 6.3 x 3.15″) and weighs 1385 g (3.05 lb / 48.85 oz) (including batteries).
Below you can see the front view size comparison of Canon G1 X III and Canon 1Ds MIII. Canon G1 X III is clearly the smaller of the two cameras. Its body is 35mm narrower, 82mm shorter and 29mm thinner than Canon 1Ds 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 1Ds MIII doesn't. Once you put a lens on 1Ds MIII, its thickness will increase significantly.
Here is the back view size comparison of Canon G1 X III and Canon 1Ds 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 (986g ) than the Canon 1Ds 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 1Ds MIII, the total kit will weigh much higher.
Both G1 X III and 1Ds MIII have weather sealings in their body, making them resistant to water and dust.
LCD Screen Size and Features
Canon G1 X III and Canon 1Ds MIII's LCD screens has the same diagonal size of 3".
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. Canon 1Ds MIII's LCD screen is a fixed type which provides almost no flexibility in shooting positions compared to G1 X III.