Um, that's not quite how it works. If you're going to look at the surface area, you need to look at the contact surface area of both objects, not just any surface area of either. The size of the wall and boulder really don't matter with this, what matters is the area where the two objects actually contact each other.
Now, if we assume a perfect world and we have a spherical boulder, then the actual contact area really depends upon the hardness of the boulder and the wall: what area of the boulder contacts the wall depends upon how much either object compresses upon impact (and then the size of the boulder too, because of its curvature. The wall only sets the upper limit of the area, it doesn't actually affect the contact area). However, since we're assuming a perfect world, everything is perfectly solid and doesn't compress, and our perfectly spherical boulders results in an infinitely small contact area (because it's the edge of a circle (which obviously has no flat surfaces) touching a straight line). Changing the size of the wall only changes the upper limit of the contact area (since you can't have a contact area larger than the surface area of one object). With a spherical boulder, the only way to increase contact area with infinite hardness is if the wall has a cavity of the exact same curvature of the boulder and the boulder lands perfectly in it. In that case, with a matching hole and boulder, then the contact area is the same as the inside of the hole (instead of infinitely small).

The reason angles make things like walls and shields more difficult to penetrate is because the projectile isn't hitting straight on, and so not all of the energy transfers into the shield. If you have a projectile traveling horizontally and a shield at a forty-five degree angle, only half of the energy from the projectile will go into the shield. More in-depth, (I believe) you only get a percentage of the x and y components of the velocity (equal to one hundred total). So depending on the angle of the shield, different percentages of your x and y velocities will be used to penetrate the shield (more x if the shield is vertical, and more y if it's more horizontal.
Unfortunately, this also means that angle isn't always a beneficial thing. At short range, projectiles are normally coming pretty much straight on, and so an angle is helpful. However, at long distance, the projectile itself is coming at an angle because of it's trajectory. So, by slanting a wall back, you're actually decreasing the relative angle to the projectile than if the wall was vertical, meaning that more energy will be transferred because of the slant rather than less.

Sorry for the long rambling. I was stuck waiting on my mom to finish tutoring and I'm my phone.

I believe it's mine, but I'm going to be inactive for a little over a week, so I won't be able to get back to the RP until then. Sorry I didn't give you guys a heads-up earlier.

Annnddd I was gone for a month exactly. Sorry, I hadn't planned on being so busy. xD Ready to begin battle again, Khan Noonian Singh ? I'll make my move.

IC: *The trebuchets fire another round of boulders against the wall, again focusing on one point on the northern outer wall. By this point the wall is severly damaged and near the breaking point.*

*As soon as the enemy archers reveal themselves, several hidden archers release precise individual shots on them and disappear into the shadows. Few elves fall in the process.*

OOC: I should have clarified this earlier, in case you've been misled: the trebuchets are out of range for your archers.