Different running footer on first chapter page

I have been stuck with this issue for a long time. The first page of a new chapter had a slightly larger font size page number.

Q: Why the first page of a new chapter is using a different running footer?

A: From one of Guido Sette TeX files:

Some LATEX commands, like \chapter, use the \thispagestyle command to automatically switch to the plain page style, thus ignoring the page style currently in effect.

To customise even such pages you must redefine the plain pagestyle.

You could do this by defining the \ps@plain command, but fancyhdr gives you an easier way with the \fancypagestyle command.

This command can be used to redefine existing pagestyles (like plain) or to define new ones, e.g. if part of your document is to use a different pagestyle.

This command has two parameters: one is the name of the pagestyle to be defined, the second consists of commands that change the headers and/or footers, i.e. fancyhead etc.

Changes to headrulewidth and footrulewidth are also allowed.