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So, as the question states, on steam locomotives with six driving wheels, the rear wheels are farther from the middle wheels than the front wheels are. You can see it in these pictures: enter image description here enter image description here

It's sometimes less obvious, but this detail always appears to be present, even if it's really subtle and hard to see. In this picture, if you look closely, you will see that the rear pair of wheels is slightly farther back than the distance between the first two pairs of driving wheels. enter image description here

I've noticed that this doesn't appear to be the case for locomotives with eight driving wheels. (Alternatively, it is present, but it's super subtle and I haven't noticed it yet.) enter image description here enter image description here

So, what is the reason for this and why does it only seem to apply to steam locomotives with six driving wheels?

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    $\begingroup$ It looks to like this could be due to the weight distribution of the engine wagon. The engine secrtion part seems heavier and needs to accommodate more weight and the weight is distributed more evenly. $\endgroup$
    – NMech ♦
    May 28, 2021 at 1:51

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In the ones pictured, there are no trailing axles - they are a 2-6-0 and a "tenwheeler" 4-6-0 respectively. The ones shown have the firebox between the two rear drive axles. .That was an evolution of the earlier 4-4-0 arrangement. In later 4-6-0, "Prairie" 2-6-2 and "pacific" 4-6-2s and tank engines with those configurations, the firebox was behind all of the drive wheels and they were as close together as possible.

Early 4-4-0 showing firebox location.

And a 4-6-0 with mid firebox

And a 4-6-2 with aft firebox.

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