When you enter a new game, Iron Golem is resummoned using the last item you summoned it from, at your current skill levels. This means if you summon Iggy with 20/20 golem/mastery, then take off all your gear and drop down to 1/1 golem/mastery, and then leave the game, when you rejoin your Iron Golem will spawn at the 1/1 golem/mastery levels. Moral of the story: always equip your prebuff gear when you leave.
Moral of the story #2: Make sure you have points in Iggy, or don't bother summoning him because he won't carry over between games. So don't burn that nice piece of gear to summon Iggy from Metalgrid.