Capitals can be made a great deal bigger than the x-height would suggest. I seem to remember the figure of 14x x-height coming up in conversation as an upper limit (seriously!) for Ornamental Penmanship, and without going berserk there's certainly scope for scaling the capitals up.
You are right to imagine large, decisive arm movements from the shoulder for OP, but textbook Spencerian is a more restrained affair and you won't find much emphasis on it in the Spencer Brothers books - it's the muscular movement resting on the forearm
with some finger movement which they promoted as the most useful for speed and accuracy. Which is to say that although the big heavily shaded capitals are dramatic, it's by the lower case and plain penmanship that you'll make your living 90% of the time.
Perhaps the smaller x-height for Ladies Hand is to compensate for the longer ascenders and descenders?
That would be my guess.