"I have no problem imaging that it is possible that a plane enters a building and leaves a hole practically the size of the plane (WTC)."
The hole - measured from severed column to severed column - is definitely smaller than the plane. The wingtips and the tail can't sever the perimeter columns.
"I have no problem imaging that it is possible that a plane hits a building and only leaves a small hole"
It's 90+ feet. How is that "small"?
"(and somehow the rest of the plane vaporized)."
It didn't vaporise. If memory serves me correctly, this claim was initially made by Le Monde, hardly an official US government body.
"at the WTC there was no exit hole."
On the contrary, there were exit holes, for example:
"some aircraft parts ... passed entirely through the buildings, landing some distance away."
http://911research.wtc7.net/wtc/evidence/aircraft.html“The fuselage crashed through 310 feet and the outer three rings of the renovated section of the building known as Wedge One”
The fuselage (if it really was the fuselage, "debris flow" would be more accurate) passed through not only part of Wedge One, but also Wedge Two.
So there's no contradiction.