My qh has flat back. My paint however has visible spine (I mean, not flat, but kinda smooth hill looking on back). They are fed same, and paint certainly don't look thin or anything (has filled sides and big butt). Even when she was overfed last year (we thought she was truly pregnant until vet came out), she had the non-flat back. My feeling always has been that fat horses should have flat back. Is that true or really depends on conformation etc.?