It's the size of the object and electrical continuity. A large piece of foil will arc, particularly if it is crinkled or wrapped around something as there will be large differences in potential between points on the conductor that are electrically far apart.

A spoon is electrically relatively small at the 13cm wavelength of the microwave, and it does double back on itself or have any large electrical discontinuities, so it would only arc if you positioned it so it got close to the sides or another conductive object.

Depending on the metal its made from though, it might get hot as all bleeding hell from the huge eddy currents that are induced, so don't grab the thing straight out of the microwave.