This is a common comment at exhibitions, almost as common as asking if it is EM or P4!
I have a theory why S is perceived as “the right size”, and it also applies to Gauge 1 in its (proper) 1:32 format. The human eye with the help of the brain measures things via angular displacement, binocular stereoscopy and perspective and as with standard geometry is pretty good at halving angles. If you take a half, and halve it, and halve it, and halve it, you have 1/32nd of your starting point: do it once more, and it is 1/64th of the original. It therefore “fits” just right into the mind’s eye. It’s only a hunch, but there is probably a PhD thesis that could come out of it! Incidentally, this also makes the scale ideally suited to the modern computational era: two to the power 6 is 64, or 100000 in binary….
So, the fact that it suits the eye is probably why people ask if it is EM or P4 – in the mind’s eye, this is the size their 4mm scale should be!