Can anyone tell me how to make buttercream balloons on a cake?

Just use a big round tip, like a #10 or #12 and squeeze making a mound of icing...make it more of an oval than a perfect circle though, you can also take a smaller tip and add a little on the end of the oval (like the knot on the balloon), then add strings!

I knew how to do them, but always wondered how walmart got them to look so full and perfect...maybe the icing? real fluffy?

I use a #10 tip, or even better, an open coupler. Hold the bag 90 degrees from the cake surface (straight up and down) squeeze the bag until the balloon is as big as you want it, then I usually finish with a flick down of the wrist as I release the pressure on the bag. Looks like a really big shell really. Then I add the strings. I also throw on a few sprinkles. Sprinkles make balloon cakes look more festive.

Just grab a piece of parchment paper and practice practice practice. Experiment with the angle at which you hold the bag, how much pressure you use, etc.