i found html reports could be reduced with the following:
1. replace css tags with divs
2. tables could be replaced with divs.
3. minimise tabs - remove tabs - minimise html formatting
4. remove (need to be verified its possible or not...)
I tested by editing html files - find/replace i achieved 10 times less file sizes, without compromising on its aesthetic design value.
In my sample report, having thousands of lines - e.g. open order lines, the file size hit 31 MB. I estimated would be reduced to 3 or 4 MB.