I have a sorcerer with a Gem Scarab familiar planned for a campaign starting at level 10, which would give the familiar 10 HD, which is more than enough for a Gem Scarab to advance a size category. This brought me to a couple issues:
Does advancing a size category retroactively remove benefits its Monster Manual entry gave it for being small and/or provide any other changes, and
Does the increase in HD due to being a familiar warrant an increased size category?
Also, would the increased Intelligence change anything else about its stats compared to what was presented in the Monster Manual entry?