Suggestions For My Breeding Project?

[ToiletWater13]

I am trying to breed some Tarpans with a high body size percentage (40%+) to make it a little bit easier to breed Mongolians. I've been mixing with Grade Horses that have a higher percentage, and also with Shetland Ponies (as they have 80-100% Body Size), but I'm still getting mixes with relatively low percentages. I was wondering if anyone had any suggestions for me on making this happen?

[Fox13]

I am trying to breed some Tarpans with a high body size percentage (40%+) to make it a little bit easier to breed Mongolians. I've been mixing with Grade Horses that have a higher percentage, and also with Shetland Ponies (as they have 80-100% Body Size), but I'm still getting mixes with relatively low percentages. I was wondering if anyone had any suggestions for me on making this happen?

Perhaps your best bet is to find an already established line (of whatever breed), that favors throwing higher sizes. Sometimes the draft breeds favor throwing larger over smaller. Forest Horses seem (in my experience from AC stock) to favor throwing medium horse types.

But an established breed that you can look at the pedigree and actually see the offspring will give you deeper insight into what that line offers. A stud that has a lot of offspring from a lot of different mares should offer you what you need to see.
What you'll need to see is offspring that offers a higher body size than their dams. Even if the stud doesn't have a very high body size, what you're looking for is the tendency to offer higher sizes, not necessarily a horse that already has it. In essence, you want to force your line into leaning toward favoring larger body sizes.