For example - I create a 1 x 1" canvas. Export as jpg or png. Reopen file in photoshop. The canvas is now 4.167 x 4.167". No matter what the canvas size, photoshop has started multiplying it by 4 upon export.
This just started happening. I reset all preferences and the problem persists. Any help would be appreciated. Thanks.
Photoshop doesn't work with sizes. Photoshop works with pixels.
The print size (the size you refer to) is determined by the pixels per inch number (ppi). This can be assigned arbitrarily, and the resulting print size varies accordingly. But the file itself is unchanged! Ppi is a measure of pixel density on paper. Pixels per inch. It means exactly what it says.
Now, Export will actually strip the ppi number altogether. It's not relevant for web/screen, so it's just discarded. The file has no ppi at all out of Export.
But when you reopen the file in Photoshop, a default ppi number of 72 is assigned. This is because Photoshop needs some number for other reasons. Any number will do. So 72 it is.
And that's the full explanation for what you see. The file is unchanged, but a different ppi number gives it a different size.