Originally Posted by Guessed

Unfortunately, different browsers interpret html in different ways - especially when coupled with a variety of security settings and installed addins. The Visio export to html includes features that may only work with a specific browser (like Internet Explorer)

On my machine and using Chrome, the test file I had zooms down with the window size. I then opened the top level htm file and edited the tag from
<iframe src="Drawing1_files/main_2.htm" width="100%" height="100%" frameborder="1" scrolling="auto">
to
<iframe src="Drawing1_files/main_2.htm" width="200%" height="200%" frameborder="1" scrolling="auto">

Doing this gave me a much bigger graphic. Does that work for your particular problem?