Pudding boobs not so great if they are supposed to be “hot & sexy” boobs. They’re sort of… dangly.
Also less than good if there’s going to be any horsing around - pudding down the front is… um… uncomfortable, and really visible. Depending on pudding color, also unfortunate.
For realistic huge boobs, use about three layers of pantyhose filled with birdseed. Not the black sunflower seeds, but the little granular white ones. Give actress/actor lots of painkillers beforehand, and super-strong undergirding and shoulder straps to help support the weight, or their back will hate you by the end of the run.
For less-realistic huge boobs, go get a bra that fits the actor, and another huge-ass cup-sized bra (remember that your cup size should be the only thing changing here).
Put the former inside the latter, and see how much space you’ve got to fill. If you’ve got less than about 3-4 inches distance between the front of the bra that fits and the inside of the huge-cups, then foam will work well. Fabric stores will have foam for seat cushions that works great. Start with a cutlet-shaped hunk that is as wide as you want the biggest part to be, and start whittling it down, then set it in place for each boob. Voila!
If you’ve got more than 4 inches space, you’re going to need some structure in there, so you’re going to have to make some re-bar foam, essentially, so it doesn’t move around in unfortunate ways. I would suggest a combination of upholstery foam, button thread, and a superlong upholstery needle, and then some electrician’s tape or foam glue. Basically, you want to use the thread to connect the quadrants of the foam with the underlying bra, so they move together (don’t pull the thread too tightly or you get unfortunate dimples), and you want to use the foam glue to make sure that the foam itself is firmly attached to both interior bra (for connection to the actor and their movements) and the exterior bra (for… you know, not having foam boobs hanging out all over the place).
Alternatively, if you feel less-inclined to sculpt foam boobs into the correct shape, and you have a dress form or clothing dummy available, then go get that expanding ‘crack sealing’ foam. Take the smaller and larger bras and connect them to each other with something fairly rigid and easily removed later (think drinking straws or coffee stirrers). Use pantyhose to create a “floor” and "ceiling"at the bottom between the inner and outer bras, and over each “boob.” Plaster your dummy or dress-form in cling-wrap and put on the bra contraption. Spray interior with foam (probably easiest to do from interior cleavage and then let it expand up into the boobs. You’ll need to use much less of the aerosol than you think you will - it expands a LOT. You’ll still have to file off the edges from where it seeps out the pantyhose, but a rasp gets it down real quick, as does a hot-knife (that will stink) or a plain old whittling knife.
PLEASE for the love of all that is holy, DO NOT USE YOUR ACTOR AS A DRESS FORM FOR THIS PROCESS. That shit gets HOT when it does its thing.
I have totally never made fake boobs for anyone. At all. I promise.