History rant moment, sorry. (It does hate on fats too though at the end) So a full harness for mounted combat would weigh around 25kg to 30kg on the upper end of the scale, add the horses armour and you are at around 50kg at the upper limit. If we want to oush it to the limits and then include jousting armour (mind this would mean most of the horse armour is removed) we would have a harness of up to 65/75kg and lance and all included you would be looking at around 85kg, maybe 90kg at a push, for thr upper end of things. So we will be nice and round all of that after conversion to 200lbs. So add a strong knight of a larger height, let's even be nice and add a tall gent of around 6foot (which was a decent amount above average for about 15th/16th century). The upper end of where a 6foot male would weight will be 180/200lbs (80/90kg)  So we have a total added weight of 400lbs/181kg.  A horse can carry about 20% of their own body weight, though some brreds can carry more but they won't have those for people to casually ride who arent trained riders. Sadly there is no average horse to take into account for this, but a horse weighing at 1000lbs/450kg would be a light horse (a draft/draught horse can be double this). Military tended to use an ancestor to the modern draft/draught horse, which would allow up to a maximum weight of the above shown 400lbs to be carried safely. Now if you think a casual unskilled fat will be put on a 2000lbs/900kg horse then you are and idiot, they would be put on a light horse, which means that if you want to ride a horse as a random person you can not weight more that 90kg, simple as fuck!  Tldr: Don't over estimate the weight of armour,here are some numbers to explain this. A knight and fill kit still weighs less than many obeasts we see casually paraded around today.  And if you weigh more than 90kg you should not casually be riding a light horse, and whilst you can probably have places that will have larger horses for you to ride you should not push it.  Fats should not be on horses, simple as!