It's a cross or garden spider Araneus diadematus, very common in gardens and other habitats. They vary a lot in colour and boldness of the marking but usually show that light cross on the back, near the front of the abdomen.
It is a female, and in the autumn they do grow large and bulbous looking before laying their eggs. It's better off outside where it can find a sheltered spot on say a fence or tree where it could find a suitable spot for the egg cocoon. The spiderlings hatch from the eggs in spring.