This is not universally true, although it does apply to some binocular families such as the Swarocski SLCs and Meopta Meostars which use scaled eyepiece designs for different magnifications. There the eye lens diameter and eye relief diminish progressively with increasing magnification.
With large objective diameters and low magnification eye relief can sometimes be excessive but adequate eye relief with smaller formats involves some design complications.
Assuming the average 32 mm binocular had objective focal lengths of around 120 mm, the 10x32 version would need eyepieces of 12 mm focal length and, with a simple design, have an eye relief of less than 12 mm.
However, if the rear elements of the eyepiece have a longer focal length (and eyerelief), a negative field lens, or Barlow, will effectively increase the objective focal length enabling the desired magnification.
John
