why are potatoes heavy?!

Potatoes are heavy because they are dense. Compare a loaf of bread to a potato. There are lots of little holes in the bread while a potato has none (visible to the human eye). A potato also soaks in water making it even heavier.
(edit: haha I read the question pretty fast and thought you asked why the potato was actually physically heavy. Well I'll leave that part in so people can have a "haha" at me :( )

The reason you get hungry again is because potatoes are carbohydrates. Carbohydrates are the closest out of lipids (fats) and amino acids (proteins) to the purest energy form that our body uses. Therefore, carbohydrates (potatoes) are given first priority to be transferred into energy. They are also the easiest of the three to digest and so passes through the digestive system a lot faster. Faster passage through the digestive system means that your body has less time to produce a hormone called CCK which signals to your brain that you are full. Less CCK = less time you think that you are full. You initially feel full because of the first little burst of CCK that you receive and then feel empty once you run out.

Potatoes (common potatoes like Idaho, Russet, etc) are HEAVY in the starch department. Starch is a simple sugar compound. Though eating a baked potato can and does make one fill up fast, but because it is mostly a starchy food,

interesting note:
In terms of nutrition, the potato is best known for its carbohydrate content (approximately 26 grams in a medium potato). The predominant form of this carbohydrate is starch. A small but significant portion of this starch is resistant to digestion by enzymes in the stomach and small intestine, and so reaches the large intestine essentially intact. This resistant starch is considered to have similar physiological effects and health benefits as fiber: It provides bulk, offers protection against colon cancer, improves glucose tolerance and insulin sensitivity, lowers plasma cholesterol and triglyceride concentrations, increases satiety, and possibly even reduces fat storage.[53][54][55] The amount of resistant starch in potatoes depends much on preparation methods. Cooking and then cooling potatoes significantly increased resistant starch. For example, cooked potato starch contains about 7% resistant starch, which increases to about 13% upon cooling.[56]
The nutrients of the potato seem to be fairly evenly distributed between the flesh and the skin. For a medium potato, with and without the skin, nutritiondata.com gives the following:[57][58]

http://en.wikipedia.org/wiki/Potatoes

It is heavy when you can't anymore carry.

The carbs/starches.