Well the only way really of knowing how the boot will pack out is to have your foot in the shell alone. The bootfitter will asses the length, forfoot width, instep room, ankle volume, genral fit ect. This will let the fitter know how the shell fits compared to your foot. So long as its not too tight the liner will pack out to fit.
The problem comes when this is missed out and you just try on boots and see what feels comfy.
Genrally a good boot wont really be that comfy in the shop. It will be tight and will need a few (possibly painfull) rides to bed it in.
Other things you can do to cut down how much your boots pack out, is by minimising the movement of your foot in the boot. This is done by getting a footbed which supports your foot and prevents it from pronating/supernating inside the boot. The more your foot moves in the boot the more it will pack out, so anything you can do to keep your foot still will help the life of your boots.
Overall, you need to find a good boot fitter who can asses the shell to your foot, make you some footbeds, then fit the liner to your foot.