The personal inventory does affect the size of the bills. The reason I say this is my bills is higher when my sim still has her garden produce in their personal inventory, this also includes what is inside the refrigerator, or if the garden has not yet been harvested. I have also noticed a difference when they go and collect things that can be collected, and keep that stuff, versus when they collect it and sell it. The bills are always higher when I decide to have my sim keep those items as well. I am really happy that EA has included the Money Tree because it really helps in offsetting those high bills because my sim wants to keep things instead of selling everything right off.