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Seems like they designed it to look good on smaller wrists -- maybe a big version doesn't look as nice.
 
I suspect the assumption is that women will prefer the 38mm and men will prefer the 42mm. The modern buckle is fairly feminine so it's only made for the 38mm. I guess they see the leather loop as masculine and thus it's only for the 42mm. Yes, it's sexist, but that's the fashion industry for you.
 
I think the buckle to leather ratio is quite nice with the 38mm, I don't know if that would be so with the 42mm. It's more likely Apple giving exclusive options to specific models for some reason beyond our understanding.
 
Supply constraint. The modern buckle likely appeals more to women than the classic buckle and since they can't produce even enough 38mm bands, given in part that the buckle itself is unique enough to be granted a patent and built from scratch, they decided to initially market to one audience, likewise with the leather loop. Both will eventually be available in all sizes.
 
I think the buckle to leather ratio is quite nice with the 38mm, I don't know if that would be so with the 42mm.

I agree with that. The metal loop that makes the clasp might start to look funny if larger
 
I agree with that. The metal loop that makes the clasp might start to look funny if larger

Personally, I think that the clasp is already pretty big on the 38mm, especially when you see it open.

Apple-Watch-Modern-Buckle-Brown-Back-Open.jpg
 
It's a ladies strap.

Indeed it is.
While I'm the 1st one to line up to say 38mm is a perfectly fine size for a man to wear (not just justifying my purchase, lol- I really do believe that!)...
However, I've owned and purchased a lot of fine "jewelry" grade watches before & can confirm: that's a lovely, classic, and elegant clasp style... that is ONLY available on women's watches.
 
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