The javadoc on the class Thumbnailator has the following note:
---
Note: This class does not provide good support for large images. For very large 
images, it is possible for an OutOfMemoryError to occur during processing.
---
I suggest removing this note from this class and just tell that this class uses 
Thumbnails.of... to produce images.
And at the class Thumbnails you can say that it has no good support for very 
large images; why? because the whole image file has to be loaded into the heap 
memory ...
-----
Thanks for the great library!