I think I do mean American.By western you mean American right? European girls aren't that much obsessed with gym , they go over there of course but mostly for shape reasons rather than build muscles , besides it's a place to meet people an example is Valentina Nappi
Anyway even in the USA it's not always true some of the most in demand girls in American porn aren't that muscled , furthermore haven't any big tits nor a big ass , I'm talking the likes of Riley Reid for example or how in demand was Alina Li who had a very similar body type even though she was Chinese but I can add more , Eliza Ibarra is another one , mind you I'm talking the "naturals" who have no need of any implant to be successful.
I like women of any kind , most of the times I prefer the regular type over the "beautiful" ones usually for minor features, their smile or their attitude , just the shape of their ass , I mean shape and proportion it doesn't have to be big , a slender woman could be as well proportioned as a tall one , you don't need to be tall to be well proportioned , say Elsa Jean or Gina Valentina for example
As for the "caucasian model" I think variety may have a great role with that . Whites can come with any nose shape, hair color, hair shape with different natural hair styles such as the natural curly ones, eyes color , any combination with the two, even their skin shade can change a lot, they can have freckles , little red or brown dots all over their body , not to mention what the mixed ones can be . All this variety looks also perfectly natural whether blue eyes or green eyes on any other woman or man look weird no matter how beautiful they are. They can also vary a lot with their body type, they could be very short or very tall even on the very same family group
This much variety can basically match any sexual taste anybody can have either male or female , this is possibly why they're seen as a beauty model , though not everybody likes that kind of girls or men , not even the whites themselves
As for white/caucasian women, I do mean what I said earlier. I know that there is variety since I lived in Germany for 3 years and saw a wide variety of white girls from north/west/east Europe but they do have basic similarities. Also, Caucasian features are indeed the beauty standard, at least for Asians since I'm one and raised in an Asian country. Most anime characters are really Caucasians in disguise!
One who is raised in the west may not realize this or a Caucasian person him/herself may not want to admit it because of decency or other reasons.