I‘ll go out on a limb and say that mine‘s bigger than hers.
My guess is that range of motion plays into this.
i.e. I‘ve spent most of my youth at the computer and as a result my right wrist won‘t pronate very far and holding it pronated for a long time makes my forearms sore.
So I‘m wondering if there is a way to specifically strengthen those weak positions.
Yes, I‘ve been paying attention to staying relaxed and I usually take breaks when my forearms get too tense.
I‘ve recently added specific finger drills (Hanon etc. if that rings a bell) and maybe I‘m still adjusting, but specifically playing lots of staccato leaves me a bit sore and can lead to crampiness and a sensation of my finger muscles wanting to get stretched.
It feels like many of the smaller muscles in the hand get exhausted quickly by these exercises and they aren‘t much involved in deads, chins and other exercises that improve grip strength.