It can be so difficult to tell. My 16hh 9yo ISH m/w type weighs between 550kg - 580kg depending on winter/ summer fit/ unfit etc. (weights taken using vet's weighbridge not weightape)
My 15.2 Appaloosa weighs 510kg and my 15.2 hann x sec d with far more bone than the appy weighs only 480. Both condition score at a perfect 3 and were on the weigh bridge last week. It is so hard to tell just by looking.