As a horsewoman, that first picture offends me too.
However, I don't have the tiny minis and I've always based the riding on the size of the mini/pony to the size of the rider AND WHETHER OR NOT THE RIDER HAS SOME LEVEL OF FITNESS TO BE ABLE TO CONTROL THEIR RIDING/MOVEMENT while in the saddle. Personally, if the pony or mini is too small to wear a saddle that would be appropriate to the rider, the rider is too large.
At different stages, I couldn't tell you what my children weighed as I don't remember - but their feet generally didn't hang down that far. I DID base the riding on their sizes AND their ability to MOVE with the pony. I once had a boy that weighed the same as our oldest daughter, ride our pony while giving "pony rides" at our neighbor's Barbecue. He was several years younger and much shorter - so his weight was more compact thru his body. However, he also had little "fitness" and no control over his muscles. After struggling to get him into the saddle (that was eye opening! & he "plopped"). He was pudgy and "soft" BUT his "size" still fit well w/i the same 13" western saddle that our daughter rode in. Our stallion, AJ, (45 + inches tall and once taped at 575 lbs) worked reasonably well. The boy had to have almost as much help dismounting as he did mounting. He literally had no balance - and was very upset with each walking step that AJ took at first. He gradually relaxed and settled, though. Later, he wanted a 2nd ride and against my better judgement, I said OK. AJ thought differently as the boy "plopped" again (I did cringe). After two steps, AJ promptly sat down just like I've seen many a Donkey do!! He didn't budge until we got the boy out of the saddle and clear - at which point AJ just as quickly stood up. I thought I understood - but to be sure, I had our daughter mount him and actually ride him around the yard where we were giving pony rides (partially our back yard and partially the neighbors' - in town about 8 miles away from the pasture we leased for our herd). That was the first and LAST time that AJ ever did that - over quite a number of years doing various pony rides and lessons. Several much older and larger children and even a couple of small adults rode him BUT they had fitness and control over their limbs. They knew how to balance and they had empathy. I believe AJ knew that. Their size never was an issue.
That said - one of the things that most
knowledgeable horse folk look at is the ratio of horse size to rider size. It isn't just weight/bulk but also the length of the torso and the legs and it's all about balance. I have as much problem with a tiny rider (especially a youth) up on a full size horse as I do a large/tall rider on a small pony/mini...
In European countries, the minis/ponies are built very differently. Indeed, a pony in most other countries the same size as some of our Shetlands and Miniature Horses are twice the weight and twice the substance. You can still find grown adults going to "a job" while riding that small equine - who works for a living. I'll restate that - our American bred Miniature Horses and Shetland Ponies are built completely different than their non-American bred counterparts "across the pond".
Most Miniature Horses and small Shetland ponies are simply not built for that size of a rider as pictured...
and yes, I too, run into problems explaining that that's not how you work with miniature horses and small ponies. However, our ponies and "minis" are all ridden at some point in their lives (or have been so far). In the end, for those of us who consider ourselves knowledgeable horse folk, it's education, education and more education and sometimes it's still like beating your head against the wall.
Most of our "Minis" are not that refined either. Here is a picture of one of our Shetland mares. This is Shado - our 2nd purebred Shetland foal. She was born in April 1997 - just a handful of days after we arrived in NC from MT. At the time of this picture, she was 4 1/2 years old and measured 35" at the withers. Have no idea what she measured as a "mini" - she is a registered Shetland only... Sierra turned 8 yrs old about 2 weeks previous to this pic. Again, I don't know what her weight was at the time. If you take the saddle off of "Shado", Sierra's legs will be much closer to her body and hanging further down. They DID go almost all the way to "Shado"'s knees - and neither "Shado" nor Sierra liked bareback riding (as a pair - Sierra did fine with other ponies - all larger than Shado).