** Formation does not have to be balanced by subduction.  For example, folding can shorten length, either of ocean material or by raising the Himalayas instead of quietly subducting India.  Continental plates may get subducted rather than oceanic, thus more ocean.  There are many factors, such as erosion filling the Gulf of Mexico.  Also, perhaps "ocean getting bigger" is not correct if one uses Iceland as being part of spreading — there crust grew to the surface and although there is spreading it may make more land rather than making more oceanic crust. ( )