Bitcoin Core isn't all that big. It's the BLOCKCHAIN that is 46 Gig. Given that it's the MAX size of a block being affected, not the specific block size, the XT varient won't have a noticeable effect if any at ALL on the total size for a long time if ever.
If transaction spam should increase to make the maintenance of a “full node[]” (
lottery248) yet
more burdensome then “the XT varient [will] have a noticeable effect” (QuintLeo).