PS file too big

OK, let's start at the beginning here.

A 48 MB raw file will produce a flat uncompressed TIFF of a little below 300 MB.

That's just a single channel at 14 bit depth encoded into three channels at 16 bit depth. And then you're expanding to a square format. That will increase the file size size by about 50%, so then it clocks in at 450 MB.

Anything bigger than that means there is additional content - layers, smart objects, extra channels, etc. So the question here is really just what is the additional content beyond a flat uncompressed file.

So now we're up to a half GB, without anything extra. What more?

How much layers do you have in your file?

Do you rasterize the appropriate layer with the KI generated background? If no, please rasterize when you are happy with the result of the generated background.

Try also to merge all layers into one and then delete all other layers.

What's the file size after doing the steps above?

That's not normal, something is wrong. A TIFF file without layers that is 1.45GB is quite large. If you're saving a single layer, there isn’t much you can do to significantly reduce the file size. Perhaps resetting your preferences could help with this?

What is the original resolution? You don't need huge images for Instagram (and they don't need to be square, BTW.) Save your TIF but then change the Image Size to something closer to 1000 pixels on the long side, and then save/export as a JPEG file.