It looks ridiculous, sorry.

What are the advertised measurements when assembled? Does that match what you received? If you find the advertised size is different from what you received, you can go after them. If the product is "as advertised" then you own it.

If you can't get a proper size (matches your mattress size) then it may be worth your while to take the headboard/foot board pieces to an upholsterer to have them take off 1.5 inches. It will be a few dollars more...but it will fit.

Do you have a regular king or a California King? There is a third "King" size bed on the N. American market = 3 different sizes. My king is from Ikea and it is slightly different than what is normally found in USA/Canada. You might want to measure your mattresses to see where things went off the rails. Start by checking advertised size "at assembly" vs. what you have in your bedroom.

Is it supposed to allow for tucking in blanket and spread?

Can you push the mattress down toward the foot of the bed? It's easy to have extra space at the head, especially if you like extra pillows, and it wouldn't be visible. Agree that a little space is needed for puffy blankets and so forth, but this looks like a lot . . .