There is no scale on that picture, so you can't really tell how large the building is. It does, however, have 3 tiers as opposed to the current Pentagon's 5, so it's likely it is somewhat smaller. Even with that, it would still spread across a very large bit of real estate - in other words, it is large
horizontally. A large reason why the Twin Towers were destroyed is because they are large
vertically. The extremely hot fires spread quickly to nearly engulf the affected floors. It also super-heated the support beams which caused those floors to collapse and then pancake down through the remaining floors. It would be much more difficult for the fire from a single plane to travel through the entire breadth of the Pentagon and destroy the entire building. Depending on where the plane hit, it's possible that one side would be destroyed, or possibly two. But unless virtually nothing was done to control the fire, it would not have spread to the other sections. Even the one side of the building that was hit in this universe, only a small part of the center that side was destroyed. Parts of that section were virtually untouched.
One outside possibility would be if the plane skimmed the top of the building, damaging and setting fire to the top one or two floors on two
opposite sides. But even with that, it's likely only those two sides would be destroyed and not the entire building.
