If new ocean crust is constantly being made, why aren't the oceans getting bigger?

According to Plate Tectonics theory, there are seven large plats on the Earth’s crust that are moving on a large scale, creating the phenomenon called continental drift. The reason why oceans do not become bigger is that the old crust is destroyed as it goes under in the subduction zones and to the mantle. According to estimates, the Atlantic Ocean is spreading, but the Pacific Ocean is not since it is undergoing subduction in many of its boundaries. 

a maximum–minimum thermometer. the scales are fahrenheit on the inside of the u and celsius on the outside. the current temperature is 23 degrees celsius, the maximum recorded is 25, and the minimum is 15, both read from the base of the small markers in each arm of the u tube. the bulbs are hidden by a plastic housing.

six's thermometer is a registering thermometer which can record the maximum and minimum temperatures reached over a period of time, for example 24 hours. it is used to record the extremes of temperature at a location, for instance in meteorology and horticulture. it was invented by a british scientist james six, born in canterbury, in 1780; [1] the same basic design remains in use.

it is also commonly known as a maximum–minimum, minimum–maximum, maxima–minima or minima–maxima thermometer, of which it is the earliest practical design.

the thermometer indicates the current temperature, and the highest and lowest temperatures since last reset.