[derefr]

One problem with this is that—at least in some Graphviz layout engines—"the size each node is going to occupy" is chosen to minimize the energy/stress of the graph, sort of like how line breaks on TeX pages are chosen to optimize for whole-page metrics.

For example, in a graph with only two nodes, where each node contains a long text string, there is no "pressure" on the nodes, so the default result might be to not break the text, creating two wide, short node-boxes, and then to arrange them with one above the other. Whereas if you have many such nodes in a densely-connected web, the text in them will likely be wrapped in order to make the nodes more box-shaped, so that nodes can fit between the cuts of the pairwise-acute spirograph-like edge lines.

In order to resolve this sort of layout, Graphviz has to understand a lot of things about fonts, padding, and borders (just like TeX does.) It's already doing "rendering" at that point, even if the result gets thrown away.