Advancing creatures by class lvs vs type

[Rackhir]

Does it matter if a creature advances by class or by type with regards to size increases? In the section on advancing monsters it just says HD and increasing by class increases HD.

 

[Infiniti2000]

You only gain size increases and racial ability improvements when you gain racial HD. If you add classes to a monster, that monster does not increase in size and none of its racial special abilities or racial special qualities improve unless the description specifically states that it does (like a drow's SR).