That plant has way too much fruit to support for it's size. You don't need to prune it, you need to let it grow more leaves, before you grow any more fruit. The tree is putting all it's resources into ripening those fruits, and not growing new leaves or roots.
Also no matter how large the pot is, you need to renew/refresh the soil at least once every year or two. Potting soil degrades and the organic elements in it are broken down and used up by the plant, or washed out by watering. The soil compacts and compresses the roots, as well as becoming much less water-retentive.
After the fruit is picked, I'd advise you to turn it on it's side and slide the whole root ball out of the pot. Wash off as much old soil as you can with the hose (maybe wait until the weather is warm) and then replace the soil with fresh, high quality potting mix being sure to fill in any air pockets.
Water thoroughly at least weekly, and about a month before it goes outside again, fertilize it with some citrus-specific fertilizer. It definitely needs water more often than every couple of weeks if it doesn't rain and the weather is hot, especially if it has fruit on it.
