There are reams of information about port sizes in a thread in the AR Discussion section. The thread is http://www.ar15.com/forums/t_3_118/641240_Official_Mk_18_and_CQBR_Photo_and_Discussion_Thread.html. In a nutshell........Colt uses .070 in the Mk18, knowing that only 5.56 mil-spec ammo is going to be used, primarily suppressed. Yes, the Mk18 is overgassed when run suppressed. Daniel Defense uses an .080 size port because they know you may run .223 ammo. It also is overgasses when using 5.56, but not when running .223 ammo.
For what it's worth, and this is just with my experience, my DD Mk18 upper is very tame when running .223 bulk Federal ammo. When running 5.56 there is a very noticable difference......it is much harsher as far as recoil...........and obviously more muzzle flash. I am also running an H3 weight buffer with an unknown spring.
Disclaimer........my DD MK18 is actually a CQBR Block II. Why they call it a MK18, I do not know. Same thing as far as operation though...10.3 inch barrel.