I downloaded a large-ish file (1.9gb) and I could not open it with either Garmin map installer or Basecamp or Mapsource. What am i doing wrong? Many thanks for the service. I have used it before and it worked fine. If only I could remember what i did!
You did not say from where you downloaded it. ZIP is a generic format for compressing files, not specific format for map data.
Historically many systems have a file size limit of 2GB. 1.9GB, of compressed data would result in 6-8GB of uncompressed data. Whilst I’m not familiar with the tools you are trying to use to read the file, it would not surprise me at al if the files was simply too large for them. This may also depend on the OS and machine.
It looks like the, presumably, uncompressed size limit is 4GB: http://www.poi-factory.com/node/38573
Aha. Size matters after all! 
Many thanks. I will attempt a smaller download