Either the Item Scale slider in the Frame Settings menu doesn't work, or I have no idea what it does.

I assumed it would scale the size of the item icons in my bags, is that not its intended purpose?

Thanks!

Oh, a little more goofing, and I see that it changes the sizes of the items when a tab other than "All" is selected..? But why doesn't it affect the all tab as well?

OK, I'm silly. The bags frame doesn't use scroll bars, so when you raise the item scale, the layout algorithm looks at how much space is available and scales to fit the available space, overriding the item scale. The item scale worked for me on the non-All tabs because they don't need to show empty bag slots, but on the All tab, the empty slots have to be shown, so the algorithm overrides the item scale to make them all fit.

Dragging the bag frame to make it bigger allows the item scale to work.