Tidy skips large files?

I use the Tidy DLL in VB.
>From the Tidy object I use the TidyMemToMem(string) method.

This goes fine for most of the pages. But when I want to tidy large pages,
(eg. www.weather.com)  it returns an empty string. 

Is there a size limit for the input string?
If yes, why? And could it be made larger?

Regards,

Arnoud Dekker
Arnoud@rosetta.nl