I've grown numerous "large" pumpkins here in the US, and can tell you that there are several factors at play in growing really big ones. One is to prepare your garden bed with well rotted manure, (I use horse or cow), and do so deeply. Pumpkin roots run deep and the plants feed heavily. Make a "hill" in to which you would deposited four to six of your seeds, covering them to a depth of about 1".
At the three week mark after germination, select the two largest plants, and snip off every other plant. From these two, allow them plenty of space to grow, caring not to snip or prune anything just yet. The goal is to get the largest, more robust vines possible, as large fruit rarely grow on wispy vines.
Once you begin to see the female flowers forming, select the two healthiest looking for your project. Upon their opening, hand pollinate both, using a cotton swab and transferring pollen from the male flowers to the female stigma. Pumpkin size is directly linked to the level of pollination achieved, so don't leave it to chance/bees.
After you've noticed that the pollination has taken, (usually two weeks to be certain), begin to remove all subsequent male and female flowers, along with any developing side branches. This will divert all energies in to the formation of the pumpkins. Given a good warm summer with plenty of rainfall, one should be able to achieve a fairly good sized pumpkin.