1. You're not too big for the pony. At your size the animal would have to be pretty small (e.g. A miniature horse).
2. I'm inclined to agree with those who said the trainer is likely meaning that you'll "outgrow" the pony for what you are going to be doing. Not that you're about to become to large for it (unless you're anticipating a massive growth spurt).
3. The "20% rule", while actually quite accurate, is only one part of the three part equation
(and there are a couple of additional variables too). For some reason it seems to be the part that everyone remembers, but it's like figuring out the area of a polygon without enough of the dimensions.
The three parts to the equation for determining if a horse is carrying too much is: Weight ration, cannon bone size (diameter), and loin width.
(the variables are things like conditioning, rider's ability, etc.... which are generally taken for granted as being up to standards).
Now for those who want to jump and say (as some of my friends did when I first pointed this out when explaining why I do what I do with my horses) "there are many endurance riders who are 30% of there horses weight and the horse does fine". To day "fine" is relative
. If you took blood samples at the end of the race and did a chemical analysis you find that there would be some elevated levels well about what would be considered normal (i.e. The horse has been "technically" carrying more than it optimally should). However, most riders give the horses a break, so they get to recover and most riders spend time conditioning the horse to carry the weight so while it might not be great for the horse on one hand, it's not going to break them down any time soon.
The weight, bone and loin criteria are really designed for horses that in fact worked harder over all than most endurance horses. Horses that had to be able to cover around 20 miles every day for 5 or more days a week and be able to do it for most of the year. Like Cav horses. You take a horse with thin cannon bones, narrow loins, load up 30% of it's body weight on it and ride in 400 - 500 miles in a month and you're very likely to put that horse out of action for any number of says while it recovers.
Since most people don't really ride that much most horses have time to recover from these things and it's not often noticed (or noticeable).
....TMI and it's not what the OP wanted to know (which is that based on the information she provide she's a long way from being too heavy or too big). I just always find it interesting how everyone knows the weight ration, but "not the rest of the story"