[perilunar]

Font sizes would be perfectly fine in millimetres. 9 pt ≅ 3 mm, 12 pt ≅ 4 mm, 18 pt ≅ 6 mm, 24 pt ≅ 8 mm. The difference is about 6%.

[WillAdams]

A 6% difference in type size would markedly change number of characters per line, number of lines per page, and pagination.