If a Sukka is above twenty, and you fill the bottom of the Sukka with pillows (that from the top of the pillows it's not twenty Amos) it's still Pasul, since we cannot consider the pillows as the floor of the Sukka. Even if you say that I'm leaving it there (Rashi for the whole Yom Tov) it's still not the floor, since it's not a normal thing to do, we say your intentions are null.
Tosfos says that this is not comparable to purses that are left in a trench between two courtyards, (according to one opinion in Eiruvin) it's considered as the trench is filled and it becomes one courtyard. (Assuming, because it only needs to be there for Shabbos, which they're anyhow Muktza and cannot be moved from there.)
If you fill it up with dirt or straw and announce that you'll leave it there, it becomes the floor and makes the Sukka below twenty. If it's dirt and you put it there without intention or straw that you don't intend to move (but you didn't announce that you're leaving there,) the Rabanan holds that it doesn't become the floor and R' Yosi holds that it becomes the floor.
The actual Machlokes was said by Ohelos. If there is a dead man in a house, the Tumah goes through the whole house, since they're under the same Ohel. But the roof protects anything that's on top of it. If the house is filled up to make it like one solid block, there is no more Ohel. Therefore, objects in the house are no longer under the same Ohel and are Tahor. But the Tuma goes straight up, and what's on the roof above the dead man is Tumei.
The Mishna says if the house is filled with dirt and straw, only if you announce that you're leaving it there is it considered as stuffing up the house.
Tosfos points out that this is not the real Girsa in the Mishna. It doesn't say anything about straw. The printer made a mistake, since this Mishna is brought here and in Eiruvin about straw. The real Mishna says if you fill it up with dirt or grain. We learn from this that straw is also stuffs up the house if announced he's leaving there Kal V'chomer from grain, which people are less inclined to leave there. We can learn that if you don't announce you're leaving there it doesn't stuff up the house, Kal V'chomer from dirt, that people are more inclined to leave there.
Tosfos points out that the Rabanan hold that dirt that you don't intend to move doesn't stuff it up (since they say only if you announce you're leaving there stuffs it up.) But it was too long to write that they argue with R' Yosi about dirt and straw that you don't intend to move and dirt that you have no intent, they left it out.
R' Yosi says that dirt without any intent is like straw that you don't intend to move and stuffs it up. If it's straw that you don't have any intent, it's like dirt that you intend to move out, and it doesn't stuff up the house.