Because it was a bug. Maybe he always appended a trailing zero byte ('\0') to some string. In general, if you're not aware of the content of the paper, you mig

Because it was a bug. Maybe he always appended a trailing zero byte ('\0') to some string. In general, if you're not aware of the content of the paper, you might want to familiarize yourself with it. In short & maybe simplified: The pre-compiled compiler contains a malware that, when compiling the compiler source, will be insert itself again. (Of course the source does not contain the malware). They used the pre-compiled compiler ("stolen" from Ken) to compile the compiler's source code; this of course inserted the malware into the newly built compiler. Then they used the newly built compiler to compile the compiler's source code again, and so on. And every time they did that, it grew by one byte because of a bug in the malware component.