Understand that boot liners--especially stock liners--will pack out and the fit will loosen. Most of the packing out occurs in the first five days or so but continues until eventually the liner has to be replaced if the shell is kept. It is very difficult for a skier, especially a relatively inexperienced skier to tell in the store if the fit is good. That's why shell fit--both length and width-- is important. One problem is that in general the higher end and stiffer a boot is the narrower it will be, although some boots come in two widths for the same model. Lower level skiers with narrow feet are lucky--they can be fitted into a stiff, narrow boot and the boot can be softened. Experts with wide feet have a tougher time--it can be hard for them to find a stiff enough, wide enough boot. Also, a boot that fits but has painful spots can be modified and most shops will do this for free up to a point for a year or so if you buy the boots there.
You didn't say what skis you have been on in the powder, but with wide powder skis there's a lot of leverage applied to the foot and any sloppiness in the fit will be magnified.
You should not have to really crank down the buckles if the fit is good, although you may have to tighten a notch more or at least micro-tighten the buckles after 5 days or so as the liners pack out