Below is a comparison of the 767, 787, and A350 main landing gear. The larger distance on the 767 also appears to make the airplane lower to the ground.
Why does the 767’s main landing gear appear spaced considerably farther apart than airplanes of a comparable size?
4$\begingroup$ I think the 767 appears to be lower to the ground because the flaps are deployed, so the landing gear struts look shorter. $\endgroup$– BarmarSep 13, 2022 at 17:25
1$\begingroup$ The apparent distance on the 767 is also skewed by the fact that the shot isn't directly head-on as it is for the 787 and A320 $\endgroup$– FreeManSep 15, 2022 at 14:15
The Boeing 767 main landing gear is not actually spaced further apart than other twin-engine wide-bodies:
|Aircraft||Wingspan||Gear Spacing||Fuselage Width||Gear Spacing:Fuselage Width|
|B767||48 - 52 m||9.3 m||5.03 m||1.85|
|B777||61 - 65 m||10.97 m||6.2 m||1.77|
|B787||60 m||9.8 m||5.77 m||1.70|
|A330||60 - 64 m||10.68 m||5.64 m||1.90|
|A350||65 m||10.6 m||5.96 m||1.78|
(Data source: Boeing Airplane Characteristics and Airbus Aircraft Characteristics)
As you can see, the 767 has the shortest gear spacing of all of these aircraft. What creates the illusion of a wider spacing, is the much smaller wingspan of the 767 and the smaller fuselage width compared to the others. You said "airplanes of a comparable size" in your question, but they are really not. The 767 is considerably smaller.
What typically determines the needed gear spacing, is the required engine ground clearance, especially when the aircraft is rolling just after takeoff or just before landing.
13$\begingroup$ I think the fuselage width also plays in. From the photos (only), using "1 fuselage diameter" as the unit of measurement, the 767 gear appear farther apart. I think the false assumption of matching fuselage sizes contributes to the OP's misunderstanding. $\endgroup$– Ralph J ♦Sep 13, 2022 at 10:07
$\begingroup$ @RalphJ Good point, I added fuselage width to the table. $\endgroup$ Sep 13, 2022 at 10:22
3$\begingroup$ Nice addition. There must be some foreshortening in the photos (767 at long distance vs the others up close, making their fuselages appear wider relative to the gear); from just the pictures I expected more difference in diameters than what these numbers reveal. $\endgroup$– Ralph J ♦Sep 13, 2022 at 10:30
1$\begingroup$ I took the liberty of adding the column that @Barmar suggested, as I was busy calculating those ratios after reading the answer, but before reading comments. Please feel free to roll back/edit as you see fit. $\endgroup$– FreeManSep 13, 2022 at 17:19
18$\begingroup$ My thought is that the camera lens is the greatest factor in the illusion of spacing. The first photo obviously uses a telephoto lens from a long ways down the runway, the other two are wider angle lenses taking photos on the ramp. This can have a dramatic effect on the perspective. $\endgroup$ Sep 13, 2022 at 20:28
The reason why the main landing gear appears to be "narrower" in the second and third photo is Perspective Distortion: when the photographer is standing very close to the airplane, the parts that are closer to them (e.g. the front part of the fuselage, the nose wheel) will appear larger than the parts that are further away (e.g. the main landing gear, including the distance between the left and right gears). If you look closely, you will notice that in the 787 photo, the nose gear tires appear larger than those on the main gear, which they are obviously not. The 767 photo is taken from the other end of the runway with a strong zoom lens, that's why the perspective is "flattened" and all the elements of the airplane are at the same scale. If you look at 787 and A350 images taken from a similar perspective, the landing gear appears spaced further apart:
(By 玄史生 - CC BY-SA 3.0, from Wikimedia)