But all these things are relative - weight differences between animals may indicate that one is fat and the other not, but there are other factors which affect it - muscle, bone, build etc.
By eyeballing, your highland looks to me to be heavier than I would be comfortable with - looking especially at her neck and shoulder., and 12 miles is nowhere near far enough to prove she really is fit in my book. Now, she may condition score ok when you poke and prod her - I don't know as I can't, obviously, - but that is why everyone on here says that condition scoring is the gold standard, rather than judging by eye, without specific points of reference and feeling for conformational markers. The only more accurate method of determining condition would surely be some hard core scientific equipment to judge % fat and scanning for internal deposits, I imagine.
I really do struggle with highland owners who term their horses fit (and even say show judges agree with them) but when you see photos and videos, the pony is barely able to trot properly and wobbling along like a jelly. And I don't mean you, OP, specifically, as I haven't seen many photos or any videos of yours.
A true, correctly done condition scoring, should be fairly objective. But you cannot condition score without having the horse in front of you and hands-on. Photos can be very very deceptive.
Showing judges and good condition scoring rarely go hand in hand IMO!
Completing a 12 mile ride does not automatically equate with a fit and well conditioned horse.
I currently own one of life's natural fatties and have come to realise that for these types grass is the work of the devil for them. I have to closed my eyes at the state of his bare field, but each time I look at him now I can see just how much better he looks for carrying 70kg less than he did a year ago.