You have asked a very intelligent question and I don't have a clue. Just kidding. Pressure is usually described/measured as PSI (pounds per square inch)think about it for a moment. If you were to push lets say 100 pounds of pressure on an area key word here of one inch square and then push another 100 pounds of pressure on the square inch next to it you are applying 100 pound per square inch of area but you are actually pushing 200 pounds. So, because a 180 grain .338 bullet has more area to push on than a 180 grain .308 bullet it will be pushed faster because of the larger area. To be exact on how much more area can be calculated by PI(3.142) X the radius squared to find the area of both bullets.
Now lets go back to my 100/200 pound example; if you were pushing 100 pounds on just one square inch, what ever you were pushing would only see 100 pounds pushing it. However, as you go up in area more pounds are pushing even it at equal PSI. A larger diameter bullet of the same weight as a smaller diameter bullet will always go faster than the smaller bullet assuming both are being push by the same PSI. I hope that helps, if I need to explain it further I would be glad too.