This drove me crazy for a while until I figured it out! Ever opened up a VDW file in Visio for your OpsMgr dashboard and it looks like this (covering only a small section of the screen)? 
This can be really frustrating especially when you need to break out binoculars to see the DA. If you press the highlighted button above the screen goes to full size but that is really annoying to have to hit it each time you go to the website. So here’s the trick:
The level of zoom that you have when you save your VDW file is the size that it will display when the VDW file is displayed. As an example, if I open the Visio diagram and set the Zoom level to 143 (shown below) once I upload this file it shows with the zoom level at the same as the level that I saved it to (143).
Summary: When creating VDW files for OpsMgr to display in SharePoint set the zoom to the level that you want it to display with on the website and then save/upload the file.