Take it from someone who has had tons of problems with the cups, these things take time to learn. Each person's body is shaped differently so there is alot of fooling around just to figure out how to place, seal, pump and juggle flaming balls.
It sounds like you're right in the middle ground of a medium and large cup. In the end, it will be good you're starting with the large cups but you'll have to practice much more with them. Like I did, noogling with just one cup for a couple of weeks will be in order to help fill out the cracks. Once you have some swelling, you can start practicing with two cups at once. For me, the trick was placing both cups on the chest and using each arm to kinda hug the cups onto the surface with the pump in a hand ready to use. By clamping on a cup's tube with the arm of the pumping hand (using your inner elbow), you can position the 2nd cup with your other arm. Now pump until you get a seal and positioned well.
At this point, you have to clamp with now free arm like your first arm, trade the pump to its hand and position the loose cup with the newly freed hand. Then pump again until sealed and positioned. Finally, you can relax your arms a bit and let the pressure equalize betwwen the cups and continue pumping if the seal holds.
It is quite a trick IMHO...
I eventually learned how to do this maneuver without trading the pump in my hands but I think it might be easier starting the first way. But there is hope, this week I have been just placing the cups... and pumping without doing any clamping.
So I tell you.. if I can do it.. you *can* do it because I'm clumsy.
DJ