As you know, yEnc encoding adds 42 to each value in the input file to shift lesser occuring characters to the critical values so that less critical values will need to be escaped.

The value 42 was taken because it's a good value on average for the kind of files posted to Usenet.

However, it is completely possible that a file, or a (large) part of a file, consists of values 214, 224, 227, or 19. After 42 is added these become 0, 10, 13, and 61 - the critical values. Each of which will need to be escaped.

The worst-case scenario is that the output is double the size of the input file.