\fs+1 is a 10% increase, not a 1px increase. Ref https://github.com/libass/libass/commit/9f2ffc03574ae323867fba00f8aaacc637bb0aa1
\fs+1 is a 10% increase, not a 1px increase. Ref https://github.com/libass/libass/commit/9f2ffc03574ae323867fba00f8aaacc637bb0aa1