Those scrollbars can be a pain. Google tries to calculate the size of your HTML before opening the infowindow (bubble). It's not always accurate, though. Here are some things to check:
1. Give all images an explicit width and height
2. Any padding, margin or line-height CSS can throw off Google's calculations
For #2 the easiest way to find out what's going on is to inspect the infowindow elements with the "firebug" plugin for firefox (or the developer tools for IE). It's usually a theme CSS setting that's causing the problem, often settings applied to the whole post body or 'content'.