Hi BB,
Of course the above is correct, however, the weight ratio of a couple of kilos to a horses body mass - that's the insignificant part.
You'll most likely find that height is probably more important because of centre of gravity around turns "half carting" and wind resistance.
Shifting of weight to spur the horse on first came about because a certain horse hated the whip and it could not be used or he'd break into a ferocious gallop.

I remember all this because it created quite a stir at the time and many thought it was a breach of rules.
Since then it's been adapted and learned by all the junior reinsmen.