In addition to what has been said, there is also the normal growth pattern for swords, like all plants. They need a period of vegetative rest. In the temperate climates plants die down and are dormant for several months (winter), or if evergreen they slow down considerably. Then they have a period of increased growth, flowering, etc. With tropical plants that remain "alive" continually, they go through periods of decreased activity as their rest.
There is also species, no mention is made of which you have, but different species attain different sizes. But as was mentioned, light and nutrients will also impact this. As long as new growth is emerging from the centre of the crown, the plant is fine. And older growth (outer leaves) will sometimes yellow and die off, this too varies.
Echinodorus species can be very variable from one tank to another. Some species will even show variation between plants within the same tank.