Happens to me too. I have to tighten up my boots more and more as the day progresses.
People talk about the liners packing out each day, then miraculously thickening back out overnight.
I don't believe it.
I think the feet shrink during the ski day, either because of the boot pressure against the foot or because of less liquid intake during the day, or both.
In my case, I attribute most of this issue to dehydration in my feet. I drink far less during a ski day than otherwise.
My feet look abnormal when I take the boots off.
My leg is its normal width above the boot/sock line, but very narrow below.
My veins stick out visibly on my shrunken bony feet.
Those feet don't look like that during non-ski days, and they don't start out that way on a ski day.