Smart objects get distorted when changing the image size

Users keep doing that.  What is that? IMO Asking for problems.  While you can change a smart object's size its like asking for some problems. Every smart object layer has an Object transform that transform the object pixels to be use for the layers content.  That transform can include distortion and warping of the object.   When you replace smart object via Replace content or open and editing the object. The layers object transform is not changed. And the existing transform was set for the original object aspect ratio and size.  There can be many transform for an object for smart object layers can share an object and each layers object transform is set for how the layer uses the object.

So while changing an object size may work in some cases the way you want in general it not something the should be normally done.  The layer may also have layer mask the have been added for the original object.  The size change could be masked off, The size change could reveal pixels in lower layers. Upper layers could also mask of the size change.  IMO its not some one should normal do.   I would think most or all mockup would not work correctly if an object size was changed.