Unfortunately, there's no such thing as a totally rational person when it comes to making nuanced decisions for which there is no objective right or wrong answer. It's simply impossible to set aside one's unconscious biases entirely. To an extent, one can work around unconscious biases by being as data-driven as possible, but for something like deciding whether to fund a startup, the decision is (at least in early rounds) largely based on a subjective read of the founders, rather than any metrics.

Note that I'm not defending any kind of biased treatment of overweight people -- I'm just commenting on the present reality of the matter.