It depends on the horse. Factors include conformation, muscle tone, fitness level, etc. It also depends on the person. A balanced experienced rider will be easier for a horse to carry than an inexperienced wobbly or bouncy rider.
I am 5'8" and 260 lbs. I ride all of our horses, from a 14h QH mix mare to a 16h long-backed paint mare, all in the 900-1,100 lb range. The long backed mare isn't as suited to carry my weight, so I don't ride her for long, just 20-30 minute schooling sessions to tune her up. She's one of my lesson horses. The 14h mare on the other hand is short backed with great muscle and has no problem toting me around as long as I want.
133 lbs is nothing for a 14-15h horse to carry, especially a Welsh. Welsh ponies usually have short backs and compact conformation, making them able to easily carry more than the 20% rule of thumb. As long as you condition your new pony/horse well and let his/her fitness build, then you won't have any problems.