It's difficult to help without an actual link we can test, with version and other configurations you may have set.
First, check your browser settings, to make sure you haven't reduced the size by default.
Some browsers keep their own font size settings independent.
Then, you must check your CSS files, either a custom CSS you have uploaded or the theme you have chosen.
There may be conflicting rules and things like that, that may overlap styles, instead of overriding them, meaning you'll get one rule applied first, then the second on top of the first one, instead of the second rule being applied replacing the first one.
Hope I'm clear...
Ramón Martins Sodoma da Fonseca
Analista em C&T
Coordenação de Tecnologias de Informação
Instituto Brasileiro de Informação em Ciência e Tecnologia - IBICT
Ministério da Ciência e Tecnologia - MCT