Let me guess, the masterminds of the thing on the last two pages of this chapter are either Luther's parents or the mayor guy who was afraid. This kind of story is so predictable, to the point of being annoying.
You'd be surprised. Remember that she's still a child so it'd be irresponsible to have her ride a horse that is too big, but a small horse cannot carry an infinite amount.
Shetland ponies, for example, are highly recommended for new riders, but they can only carry around 60kg.
To add to that, this world doesn't have a lot that we have in our world, so it could very well be that horse breeding isn't as advanced as well. We've done a lot of breeding to get horses to where they are now. Back in the time of the Egyptians, horses couldn't carry people at all. In fact, that remained true for Europe for a very long time, due to a lack of controlled breeding.
So it is perfectly reasonable that she wouldn't start riding until she's around 60kg and complain about riding with her brother, taking the total weight way over the ~110kg a normal horse can carry without being distressed.
Quite brave ruffians to wound a duke's son. They must have some backing. Though that being said, it's interesting a duke's son isn't travelling with any men-at-arms.