Trouble scaling assets for consistency on iPhone XS and iPhone XS Max [on hold]

I'm having trouble trying to get assets to scale properly between the iPhone X/XS and iPhone XS Max (also iPhone 8 vs 8 Plus). I would like the asset to scale proportionally across the different screen sizes.

My understanding is that both the X/XS and XS Max use 3x assets, but differ in display resolution: 375pt for X/XS and 414pt for XS Max. I have designed the asset on Sketch with 375pt width to scale properly for 3x on the iPhone X/XS. The asset displays properly on X/XS but stays the same size on the XS Max instead of upscaling to look proportional on the larger display.

I have also tried to resize the asset to 414pt width, but that creates the opposite result: looks right on the XS Max but too big on the X/XS instead of downscaling to look proportional.

This is what I expect from both screen sizes: asset scaled correctly

But this is what I see on XS Max (Plus): asset not scaled

Ignoring the image heights, see how the XS Max asset stays the same size instead of upscaling proportionally?

It seems like I can force the assets to scale properly on the XS Max by code, by going through every asset and manually scaling them, but I assume that it should be automatic and I'm doing something wrong.