Ok, it looks like the ones I tested this with were below the size limit. Invision Board have a hardcoded 300 post limit on their 'download topic' option. Here's their official statement:
QUOTE
If it is set too high, it is going to time out during the process and/or the file will be too large. I've actually seen sites with 1000+ pages in a topic...that's somewhere near 20,000-30,000 posts. The script would just die.
We are internally discussing ways of improving this, however at this time that is an intended limitation of the software.
I could raise the hardcoded limit but it would probably bog down the server and isn't a good idea.