File sizes of different help formats

I am trying to encourage our team to update the format of our help files from HTML to WebHelp or FlashHelp but there is an issue with the size of the output files generated for WebHelp and FlashHelp as they are much larger than the .CHM file size.

Please can you tell me why the sizes increase for the same project published in different help formats and let me know if there is anyway of reducing this?

Thanks.