If you're not a body builder with great upper body strength, how do you make large vessels? Well, I am certainly not a body builder, so I have to resort to other techniques when trying to throw large 20+ pound pieces.
Composite forms have become my friend. If you are not familiar with a composite form, it is a form made out of multiple pieces (see pics). Measure the top opening of your first form and use that measurement for the top of the second form. Once the pieces are set up, score the rims of both pieces and coat with slip. Place the second piece on top of the first - you can either cut off the bat before or after you have placed the second piece, depending on size and how well it stays in place. Smooth the joint very well and when you start shaping the pieces, push the clay down over the joint, so piece two really joins to piece one. Once the pieces are combined, you can continue to smooth and shape your joined pieces. It's a fairly simple way to create a large form, without the large muscles!