Please explain the difference b/t raw milks from

[big brown horse]

 

[Bubblingbrooks]

I thought it was butter fat content? Heritage breed have more?

Yes to that. Think about it this way. Modern day veges have been "bred" to be bigger and better. Course we all know that commercial tomatoes are plain old awful. Not much taste to them. And that huge overgrown vege does not have any more nutrition in it to match its size.
The same is true of a modern Holstein cow. Science has caused her to have an unbelievably massive udder that produces huge amounts of milk.
That milk is watered down and as such, is lower in fat. It does not compare to the milk that the original Holsteins gave.