Has anyone tried measuring their hand span from thumb tip to 5th fingertip and thumb to 4th fingertip? You may well find that the latter is slightly longer, if your ring fingers are longer than your index fingers, like mine are. (I believe that women have ring fingers shorter than their index fingers, while men tend towards the opposite). My LH measurements are 8.15in 1-4 and 8.05in 1-5. On RH, it's 8.05in 1-4 and 8in 1-5. Which explains why I find it slightly easier to stretch 10ths on LH than RH, and probably why I almost always play 1/4 on black octaves, unless it's a chord where the 4th finger is already employed elsewhere.
But it might also be because my teacher taught me to play octaves this way, from the very first piece I learnt that has fast octave scales (Schubert's D664). I remember being rather miffed that I had to roll the very first RH chord when Sviatoslav Richter (my hero at the time) could play it perfectly as written, and thought that I wasn't a 'real' pianist when I couldn't even physically play what Schubert actually wrote......
For most people, I imagine the span is pretty close one way or the other. The difference, especially for people with smaller hands, is the angle from 1-4 vs the angle from 1-5. The thumb and pinky are the two fingers with greatest lateral motion, and also are at opposite ends of the hand. When you "stretch" your hand out and look at it, you can see there is a much greater angle between 1-4 vs 1-5. The 1-5 span is nearly perpendicular to the wrist, which makes it much easier to play octaves without twisting.
For small hands, especially, a 1-4 span requires one of two things: a large enough hand that you can span the distance, or a reposition of the hand/arm (without twisting if played correctly) in order to span the distance.
Any time you have to reposition, it slows you down. Extra movements require more time and effort to execute. So, the idea should be to minimize this effort. This, ironically, was the initial reasoning behind the switch from 1-5 to 1-4 on black-key octaves. (The hand has to move in to play a 1-5 black key octave.) However, with octaves, left-right movements to reposition (for smaller hands) takes longer and creates a "destructive interference" effect with the muscles. An in/out movement creates far less interference with speed than a left/right movement. So, for smaller hands, it is usually advised to forgo the left/right reposition, and to move in/out to play a 1-5 octave on all keys.
Also, if you start in on white keys and out on black keys (rather than playing in the middle of the white key or the middle of the black key), you can minimize the in/out distance your hand has to travel.