Screenshots aren't of the correct sizes

When I take screenshots in different environments, I get screenshots of different sizes, none of which are the size that is listed in the Huxleyfile.json file. In particular, when I updated from 0.7.4 to 0.8.1, things started breaking (see [this commit](https://github.com/Khan/KaTeX/commit/824fb7fd2f5da39f65b469def167b4dbf0b4ed21), where the images changed)

Some of the environments are:
- Inside a docker using Xvfb, running on a VPS (like linode)
- Inside a docker using boot2docker, also using Xvfb
- Running directly on a mac

Is there something that I'm missing? 0.7.4 of node-huxley was so good at producing correctly-sized screenshots, so I feel like I must not be enabling an option to do the same screenshot-image-size correction that used to happen.

Start by reviewing Huxleyfile.json and the linked commit, then compare screenshot dimensions between node-huxley 0.7.4 and 0.8.1 in the listed Docker/Xvfb and macOS environments. Determine whether an option or environment difference explains the regression; done means screenshots match the dimensions specified in Huxleyfile.json.

Written by the indexing model from the issue text.