While it's hard to prove that this (or anything) directly causes headaches, Chirp has an unusually small x-height (stem width to x-height ratios would normally

While it's hard to prove that this (or anything) directly causes headaches, Chirp has an unusually small x-height (stem width to x-height ratios would normally be around 1:6 for a legible font – Chirp's ratio is 1:4, very small) and an extreme x-height to body height ratio (the capitals are MUCH taller than the non-capitals). The net effect is that even at the same "font size" you have to squint more to read anything, because non-capital text is very very small and tight relative to the rather large capitals.