Dan, it all has to do with screen resolution and aspect ratio. You monitor is different from your clients monitor. And you probably have the Dashboard screen size set to automatic. Tableau is trying to adjust your viz to the smaller resolution/aspect ratio, but it's just not possible to squeeze say a 1920x1080 into a 1024x768, at least not without some of the difficulties you're having.
There are different ways of attacking the problem. If it's for a single client, then find out his screen resolution, set your monitor to a similar resolution and re-work the viz. This works well if you can duplicate his resolution exactly. If you're not able to do this, let me know and I'll write more about other options.